DnD books (not settings) of older editions that are useful regardless of edition!

[vestcoat]

+1 for the 1e DMG.

The DMGII for 3.5 has a chapter of "archetypal locations" my players have enjoyed.

I've gotten a lot of millage out of the beautiful battle maps in the 3e Fantastic Locations series.

The Basic Book of Marvelous Magic and Pied Piper's El Raja Key's Arcane Treasury contain some very idiosyncratic items sure to shake up jaded players.

2e saw a lot of good cardboard standup's in Castles, Cities of Mystery, Dungeons of Mystery, Strongholds, and the WGA1-3 trilogy.

I'm a big fan of Carl Sargent's writing and creativity. Most of his works are modules and settings, but PC2 Top Ballista and GAZ13 The Shadow Elves can be used in any edition and the pantheon write-ups in Monster Mythology are superb.

B1 In Search of the Unknown is essentially an edition-less, classic dungeon crawl. The tricks and traps went on to become dungeon staples and cliches, but if your players haven't experienced them yet, it's well worth the ride. And the advice for beginning DM's and players still holds true today.

The Gazetteer line, the 2e Historical Reference series, and many Forgotten Realms products are invaluable for adapting real-world cultures to D&D.

The OD&D Geomorphs and the 2e Dungeon Builder's Guide are excellent.

I have a growing appreciation for Troll Lord Games. Not every product is a home run, but I'd recommend the Castle Keeper's Guide, Codex Nordica, Monsters and Treasure of Aihrde, The Storyteller's Thesaurus, and the Gygaxian Fantasy Worlds series to players of any edition. Lately I've been pulling the Gygaxian books randomly off the shelf and finding something to add to every session. Insidiae is better than TSR's DMGR1, 5, & 6 combined. Towers of Adventure is also good, like a dungeon geomorphs, monster & treasure assortment, and rogues gallery rolled into one, though the maps are hit and miss.

Finally, The Everyone Everywhere List Random Name Generator by Magic & Tactics Unlimited is the best $5 a lazy DM could ever spend.

 

[Greenfield]

One time I was perusing my FLGS which has been around for over 40 years.

In their back room (discount/discontinued section) I found some 2e setting books that were so neat I bought them to give to members of my game group for Christmas.

They were: Age of Heroes (Greek setting), The Glory of Rome, Celts Campaign Sourcebook, Viking Campaign Sourcebook, A Mighty Fortress, and The Crusades.

Several of these are referred to as "Historical reference", and they're priceless if you want to run a campaign in such a setting, or just incorporate these cultures into your game world.

They're "perfect bound" softcovers and though we don't play 1/2e any more, these were great sources for ideas, names, and the kind of detail that lends authenticity and depth to the story.

I don't know if they're available in PDF format. If your FLGS doesn't have them, check Amazon.

 

[Celebrim]

2e AD&D 'The Complete Thief's Handbook'

While the rules set is out of date, the general discussion of criminal organizations and skullduggery remains a useful introduction to the theme for any fantasy setting.

3e D&D, Green Ronin's 'The Book of the Righteous'

Again, while the rules set is out of date, the example the book provides of organizing a world's cosmology and making its different cults feel unique, vibrant, and meaningful in the daily life of the campaign world is invaluable as inspiration.

 

[Ralif Redhammer]

I absolutely love the Vikings sourcebook.

Another recommendation would be any Grimtooth's Traps book. They really do set the bar for devious traps.

One time I was perusing my FLGS which has been around for over 40 years.

In their back room (discount/discontinued section) I found some 2e setting books that were so neat I bought them to give to members of my game group for Christmas.

They were: Age of Heroes (Greek setting), The Glory of Rome, Celts Campaign Sourcebook, Viking Campaign Sourcebook, A Mighty Fortress, and The Crusades.

 

[TheMiddleAgedOne]

I really liked both DMGs for 4th, lots of good ideas for general gaming, best was Fantastic Terrain (very good examples in both books

 

[Orius]

They were: Age of Heroes (Greek setting), The Glory of Rome, Celts Campaign Sourcebook, Viking Campaign Sourcebook, A Mighty Fortress, and The Crusades.

Several of these are referred to as "Historical reference", and they're priceless if you want to run a campaign in such a setting, or just incorporate these cultures into your game world.

They're "perfect bound" softcovers and though we don't play 1/2e any more, these were great sources for ideas, names, and the kind of detail that lends authenticity and depth to the story.

I don't know if they're available in PDF format.

They're all in .pdf over at the DM's Guild:

http://www.dmsguild.com/product/110...ence-Series-2e-BUNDLE?filters=0_0_45463_0_0_0

 

[GreyLord]

I would say if you play in the Realms, the Grey Box is always useful. In addition, though not as high on my list, I also frequently check the 3e FRCS book.

In fact, a LOT of the area hardcovers for 3.5 Forgotten realms are extremely useful in regards to fluff and other information. I never read the Volo books, but if they are anything like the 3.5 region books, they may be extremely useful to someone playing Forgotten Realms as well.

The Living Greyhawk guide is also extremely useful for those playing in Greyhawk. It may have been for a living campaign, but I found it far more useful than just about any other informational book on Greyhawk. I think it does a far better job than the earlier GreyHawk adventures that came out during 1e.

 

[TarionzCousin]

I never read the Volo books, but if they are anything like the 3.5 region books, they may be extremely useful to someone playing Forgotten Realms as well.

Volo's Guides are fantastically useful. They're full of inns, taverns, shops, and NPC's in the various towns and cities of their regions.

 

[Imperialus]

City State of the Invincible Overlord (world Emperor is good too) needs to make this list. Even if you just use it for the maps.

Otherwise, (I'm not sure when it was published) but Grain into Gold is a work of art if you want to create a believable economy.

 

[Bigkahuna]

Scarred Lands "The Wise and The Wicked"
Really great sourcebook for adding some of the most interesting villains I have put into my campaigns. Its naturally for the setting but easily adjustable for other settings.

The Book of Taverns by Frog Games
This is just a series of Tavern locations that can be dropped into any campaign, the great thing is that a lot of thought went into the surroundings and its made general enough to fit everywhere but ensures each location is unique almost to the point where a tavern itself can become a starting point of a campaign.

2nd Edition Dungoeneering Guide and Wilderness Guide
These are my old stand by's. You get information about dungeons and wilderness area with a more realistic gritty feel to them, aka, less high fantasy and more reality. These books have always helped me to sell the realism in an otherwise very fantastical world.

1st edition Dungeon Masters Guide
Someone mentioned that it describes DMing in a style not used anymore, I beg to differ. I have played in great many role-playing games, in particular in D&D. The best experiances always come from the DM's who have this old school experiance, even if they are using new school methods. There is magic in this book, its a unique resource that describes DMing and offers advice in a way you will never read anywhere else. Its the bible of role-playing as far as I'm concerned and only a foolish GM would ignore it.

4th edition Dungeon Masters Guide
Of all the things I hate about 4e, both as a system and as a game, it would be a crime not to mention the 4e DMG. This book is a modern look at role-playing and while I still think every good DM should read the classics, the 4e book offers some amazing practical DMing advice. Great book, a must have in any good DM's collection.

There are a few others I would consider mandatory reading but as far as resources go, these are extremely valuable.