Your single favourite RPG book?

[Olaf the Stout]

It's a tough one. It would be between 2e Complete Thief's Handbook, Traps and Treachery (FFG), Feng Shui rulebook and the Shackled City AP hardcover.

The 2e Complete Thief's Handbook because that was the first book I bought (along with the 2e core rulebooks) and my first character was a Thief with the Assassin kit.

Traps and Treachery was probably the first 3rd party rulebook I bought. I've always had a soft spot for traps, and the book itself is a fun read of various contraptions.

The Feng Shui rulebook was my first experience with a much more rule-light RPG that encouraged the player and GM to describe off-the- wall action.

The Shackled City AP hardcover was the first campaign that I've ever run from the start to its intended conclusion around 19th/20th level. It is by far the best campaign I've ever run and the book itself is thoroughly trashed from the amount of use it got (I'm generally very, very kind to my books).

 

[Skepticultist]

The Arcanum

The Arcanum was published in 1984 by Bard Games, and was part of a trilogy of books that described the Atlantean System. The other two books, a bestiary and a campaign setting guide, are interesting, but the Arcanum works entirely without them. The Arcanum represents a transitional state at Bard Games. Eventually the Atlantean System would become the basis for Talislanta and more or less completely abandon it's roots as a AD&D hack, but the Arcanum is right in the middle between AD&D and Talislanta, and so it works extremely well as a AD&D supplement.

My favorite element of the game is the very detailed and complex alchemy and herbalism rules, which are far more sophisticated and interesting than the standard magic item creation rules. It also includes rules for creating "homunculi," but the Arcanum defines homunculi very broadly, such that it includes any creature created by magic that isn't a golem or mechanical construct. I love this system because while D&D always suggests that X or Y beastie was "created by wizards," there have never been rules for that particular feat. The Arcanum supplies those rules.

I also really love the Arcanum's skill system. It's the most naturalistic system I've ever seen, and it doesn't allow itself to get locked into "everything must work the same way" that is so en vogue now. Some skills -- like Drinking -- are never rolled, they just give bonuses to other rolls (Drinking, for example, gives a bonus to saves to avoid the effects of alcohol), or allow the players to automatically do some thing (Catwalk allows a character to move their full movement along narrow ledges and rails). If you take one of the various Artistic skills, you roll randomly to determine your initial talent, and you have a random chance to improve with each level gain. Two players might take the same Music skill, and one will start out a world-class virtuoso who simply can't improve (they are already Beethoven), and the other player might start out unable to play a simple tune without offending the audience and never improve.

It also has a lot of new and interesting classes, several different types of spellcasters with vary different spell lists, and a bunch of new races.