Tome of Tomes, Volume 1.
Now, technically this is aimed at the Old School, D&D rooted rules and clones, but there really is no reason this couldn't work just as well with 3.5 / Pathfinder or even Tunnels & Trolls. Any fantasy based RPG that has folks with dangerous weapons and spells looking for loot should find this right up their alley.
Joe does a great job not only detailing the details of the tomes in question (size, weight, author, materials, value, fields of study and the like) but he also gives each tome a paragraph that deals with background and other fiddly bits. These books live and breath. I would have loved to have had some of these in my 12th level wizard's study back in the day.
A hundred books for three bucks. Hard to beat that.
My personal favorite? Beardless Ladies - Kinda like a Playboy for Dwarves. I suspect there is a copy hidden in the Tavern's Privy ;)
From the blurb:
Books in fantasy games have always been sources of mystery. We're all familiar with the sight of the magic user's eyes lighting up like a 5-year-old at Christmas when one is found. This work provides 100 tomes for your gaming pleasure, ranging from the magical to the mundane, from the common to the unique. Never again will you be at a loss to describe that recently found libram.
Tome of Tomes is a compilation of books that could be found on a sage's shelf or on the shelves of a city library. Each tome is described and classified according to field of study and special knowledge category (if applicable).
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