Glyfair
Explorer
A pretty regular thread theme I see here and on the WotC Eberron forums are the "what books should I buy to get into Eberron." I thought I'd try to put together a definitive listing to save some wear and tear on my (and others) fingers 
In my opinion there are three rating necessary for each product. First is the use for players. Players typically are interested in thing for their characters. That means a basic idea of the campaign world, crunch (feats, races, classes, etc.) and ideas that lead to character concepts (details on the dragonmarked houses, for example).
Second we have the DM. He's less interested in the crunch elements, interested in more background for the world and story ideas and typically can use adventures and adventure ideas and tools.
Finally there are those more interested in the background. Some are players more interested in the story elements, some are DMs, but a large number are the "readers", those who rarely get to play but want to read about the world.
Right now this is a work in progress. Feel free to include your own opinions and rankings.
Eberron Campaign Setting
Player: ****
DM: *****
Background: *****
Almost indispensable as it has the core rules for the setting, information on the races and the storyline foundation of the world.
Players can get most of the information from other books, so might be able to avoid it. However, since everything is in one nice package, I don't recommend this unless your DM likes to keep tight control over any campaign "secrets" and doesn't want players to possibly read anything their characters don't know.
Sharn: City of Towers
Player: ***
DM: *****
Background: ****
Possibly the strongest book outside the campaign setting. There is a limited amount of bits for players, but the detail on the city is still useful. The city is well detailed, in the Eberron style. By that I mean they give good general information, but don't break down the city too far. For example, the overall city layout is covered, but they don't restrict your game by giving detailed maps of each area.
This should be part of every Eberron DMs books, unless they have no intention of running in the area of Sharn.
Races of Eberron
Player: ****
DM: ***
Background: ***
If you are going to run a non-human in Eberron, this book is highly recommended. It gets heavily criticized for certain racial backgrounds (with changelings standing out for the criticism) but has it moments (the kalashtar chapter). Still it's a bit uneven for background. For character creation, it's quite strong.
Five Nations
Player: **
DM: ****
Background: ****
Strong on background, but light on rules. Players will mostly be interested in the character classes (one per nation), but DMs will get the most use of this. Of special note is the Cyre/Mournlands chapter.
Explorer's Handbook
Player: **
DM: ***
Background: ***
This book suffers from a slight focus problem. I thin both players and DMs can get use out of it, but neither fully utilizes it. The players get almost no use out of the large section of adventure locations, so I call this a DMs book.
Magic of Eberron
Player: ***
DM: ***
Background: **
I admit, I'm a bit prejudiced against "Grafts of Eberron." I feel that large sections of this book are taken up by topics that aren't going to be used by many groups. Too much space is taken with grafts, draconic prophecy feats, etc.
That being said, tehre is still a lot of rules elements that players & DMs alike will get use of. Plus, if you are a graft fanatic, tehre is plenty here for you.
Deluxe Eberron Dungeon Master's Screen
Player: *
DM: ****
Background: ***
Sure, having a DM's screen with Eberron art to add atmosphere is nice. However, the big selling point is the large detailed map of Khorvaire. You can download it online, and take it somewher to be printed. However, this is much cheaper, and you get a DM Screen in the bargain.
Deluxe Eberron Character Sheets
Player: **
DM: **
Background:
These days printed character sheets are of limited use. Most have their own custom sheets, and even those who like these can download variations and print them up.
However, if you have limited computer access or just like the feel of character sheets, these do the job.
Player's Guide to Eberron
Player: *****
DM: ***
Background: ****
The biggest issue I've heard about this is the number of "world secrets" contained in the knowledge sections. Personally, I think it's a losing proposition to keep players away from such information, especially if they read the novels.
This is a great tool for new players, as they can quickly look up an entry at the table to see what their characters might know about a subject. There is even a bunch of interesting crunchy bits in the book. Unfortunately, it is very scattered and finding the bit you want can be a chore.
Secrets of Xen'drik
Player: **
DM: ****
Background: **
This has a few classes and feats PCs might use, and a bit of background on Xen'drik and Stormreach. That's about as far as the player use goes. However, it's a great adventure toolkit for DMs to build their own adventures in Xen'drik.
Faiths of Eberron
Dragonmarked
Secrets of Sarlona
The Forge of War
Novels
Tales of the Last War
The Dreaming Dark
The Lost Mark
The War-Torn
The Dragon Below
Blade of the Flame
Heirs of Ash
The Inquisitives
Comic
Eye of the Wolf

In my opinion there are three rating necessary for each product. First is the use for players. Players typically are interested in thing for their characters. That means a basic idea of the campaign world, crunch (feats, races, classes, etc.) and ideas that lead to character concepts (details on the dragonmarked houses, for example).
Second we have the DM. He's less interested in the crunch elements, interested in more background for the world and story ideas and typically can use adventures and adventure ideas and tools.
Finally there are those more interested in the background. Some are players more interested in the story elements, some are DMs, but a large number are the "readers", those who rarely get to play but want to read about the world.
Right now this is a work in progress. Feel free to include your own opinions and rankings.
Eberron Campaign Setting
Player: ****
DM: *****
Background: *****
Almost indispensable as it has the core rules for the setting, information on the races and the storyline foundation of the world.
Players can get most of the information from other books, so might be able to avoid it. However, since everything is in one nice package, I don't recommend this unless your DM likes to keep tight control over any campaign "secrets" and doesn't want players to possibly read anything their characters don't know.
Sharn: City of Towers
Player: ***
DM: *****
Background: ****
Possibly the strongest book outside the campaign setting. There is a limited amount of bits for players, but the detail on the city is still useful. The city is well detailed, in the Eberron style. By that I mean they give good general information, but don't break down the city too far. For example, the overall city layout is covered, but they don't restrict your game by giving detailed maps of each area.
This should be part of every Eberron DMs books, unless they have no intention of running in the area of Sharn.
Races of Eberron
Player: ****
DM: ***
Background: ***
If you are going to run a non-human in Eberron, this book is highly recommended. It gets heavily criticized for certain racial backgrounds (with changelings standing out for the criticism) but has it moments (the kalashtar chapter). Still it's a bit uneven for background. For character creation, it's quite strong.
Five Nations
Player: **
DM: ****
Background: ****
Strong on background, but light on rules. Players will mostly be interested in the character classes (one per nation), but DMs will get the most use of this. Of special note is the Cyre/Mournlands chapter.
Explorer's Handbook
Player: **
DM: ***
Background: ***
This book suffers from a slight focus problem. I thin both players and DMs can get use out of it, but neither fully utilizes it. The players get almost no use out of the large section of adventure locations, so I call this a DMs book.
Magic of Eberron
Player: ***
DM: ***
Background: **
I admit, I'm a bit prejudiced against "Grafts of Eberron." I feel that large sections of this book are taken up by topics that aren't going to be used by many groups. Too much space is taken with grafts, draconic prophecy feats, etc.
That being said, tehre is still a lot of rules elements that players & DMs alike will get use of. Plus, if you are a graft fanatic, tehre is plenty here for you.
Deluxe Eberron Dungeon Master's Screen
Player: *
DM: ****
Background: ***
Sure, having a DM's screen with Eberron art to add atmosphere is nice. However, the big selling point is the large detailed map of Khorvaire. You can download it online, and take it somewher to be printed. However, this is much cheaper, and you get a DM Screen in the bargain.
Deluxe Eberron Character Sheets
Player: **
DM: **
Background:
These days printed character sheets are of limited use. Most have their own custom sheets, and even those who like these can download variations and print them up.
However, if you have limited computer access or just like the feel of character sheets, these do the job.
Player's Guide to Eberron
Player: *****
DM: ***
Background: ****
The biggest issue I've heard about this is the number of "world secrets" contained in the knowledge sections. Personally, I think it's a losing proposition to keep players away from such information, especially if they read the novels.
This is a great tool for new players, as they can quickly look up an entry at the table to see what their characters might know about a subject. There is even a bunch of interesting crunchy bits in the book. Unfortunately, it is very scattered and finding the bit you want can be a chore.
Secrets of Xen'drik
Player: **
DM: ****
Background: **
This has a few classes and feats PCs might use, and a bit of background on Xen'drik and Stormreach. That's about as far as the player use goes. However, it's a great adventure toolkit for DMs to build their own adventures in Xen'drik.
Faiths of Eberron
Dragonmarked
Secrets of Sarlona
The Forge of War
Novels
Tales of the Last War
The Dreaming Dark
The Lost Mark
The War-Torn
The Dragon Below
Blade of the Flame
Heirs of Ash
The Inquisitives
Comic
Eye of the Wolf
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