In feudal Europe, a book was worth about the same as a small farm. Hand-written, usually illustrated (as a high-price item, might as well go all the way!), variously bound, these objects containing knowledge that required no teacher were precious. A library was a sign of wealth, education, and power.
When you add ancient mysteries, lost cultures, scholarly works, grimores of spells, and research into supernatural power into the mix then you have books that serve even better as treasure.
I was always captivated by the idea of wizards adventuring to find scrolls and spellbooks with magic they did not yet know, to expand their power. The idea of looking for a scholar’s shocking work in a lost book is neat. You start mixing Cthulu mythos tomes with their sanity-blasting knowlege into the mix as a model, and books get pretty exciting.
In my geomorph dungeon stocker, in the loot section, I have a variety of different books; journals of adventurers, treatises on otherdimensional locations, a book that contributes information on a gap in historical knowledge, and that sort of thing. We haven’t even gotten to books with magic effects yet.
Also, think about how the books should be found. A chain library, with a loop of iron depending from each book spine and a chain down the row, so no one can steal one. Books with locks, possibly magic locks, and protected hinges. Scrolls so brittle they crumble beneath the touch, but must be removed in an athletic way from a dangerous resting place.
A quick way to discourage the worn cliche of a homeless adventurer is to start giving them books they’d rather keep than sell. They have to put the books somewhere, inside, protected. Might as well sleep nearby.
This is also a loot item the characters can MAKE. Consider the Red Book of Westmarch, Bilbo Baggins’ record of his journey There and Back Again. If the characters faithfully record jotted maps, notes on monster strengths and weaknesses, out-of-place elements of their adventuring, sketches of the glyphs on the wall of an escape tunnel of a long-dead civilization… not only can that become more loot as the relevant knowledge is connected to their raw data, but that also adds a persistence and continuity to the setting.
One way to encourage this is to ask players to make note of what goes in the book. The players don’t have to create the book themselves; this is generally too much to ask of players. But if they have a point list of what they’ve recorded, then if the ideas are slow coming sometime, you can get your hands on it and review it for hooks.
Don’t forget to make these rare objects interesting. Reptilian skin, never-cold brass hinges, a daemon bound into the meat of the book so the cover seems to writhe and scream, a coral spine with the pages tied around it–then there are scrolls, and folios, and so forth.
Size matters. For books on stands that were not meant to leave a library, make them big! For a rogue’s spellbook for the two spells he learned, make it pocket sized. For a monster hunter’s record of experiences, put each expedition on a piece of the adventure–hide, a broad bone, a withered but huge leaf, etc.
In generating book loot, efficiency can be the enemy of effectiveness. This is a great place to go all gonzo on your players. The better they remember it, the better a story it will someday be.