Isn't it funny how over my holiday vacation, with plenty of time on my hands, I had nothing in mind to post about. As soon as I'm back in the saddle at work, I can't wait to start posting again...
Anyhow, books. Most fantasy campaigns have them, in one form or another. Some of them are magical, but most of them are really just plain old books covering an endless variety of subjects, from the history of ancient kingdoms to the heraldry of various noble houses, to descriptions of battles won and lost or how to identify various plants and their medicinal or Alchemical properties. In any event, books are a rare and valuable commodity in most ancient cultures before the advent of the printing press, and some are probably easily worth their weight in silver, if not gold.
And yet, despite the treasure trove of knowledge and learning available in these books, most systems lack any sort of mechanic for "book learning". If a PC picks up a book on the history of the Argothian Empire and spends a few weeks reading it, most systems, as written, do nothing to show that the PC has acquired any new ability to use that information. The GM can, of course, make a spot rule or ruling that says the PC now has X% chance to answer any question on Argothian ancient history, but typically this is something the GM has to intervene on.
Call of Cthluhu had a mechanic in place with regard to "Mythos tomes" and how they raised a PC's Mythos skill, and I think it worked pretty well. Each book had a percentage rating for how far it would raise your Mythos skill, and the time it took to digest the information (as well as how much of your precious Sanity it would burn away...). However, this mechanic was for Mythos tomes only, not "book learning" in general.
I can understand that most RPGs are about "adventure" and therefore creating rules for sitting around and reading books seems a little...lame. On the other hand, setting up a few basic rules to get more out of books and libraries in game might actually be beneficial to the PCs. In the TV series Buffy the Vampire Slayer, most of the discussion of how to take down the Big Bad Guys revolved around what could be dug up in the school's library, and the character of Willow, the spacy nerd who did all the digging, played a vital role in defeating a lot of the bad guys (even before she began practicing magic).
So there is a good precedent for being able to weave "book learning" into an RPG while still maintaining an action-y pace to the game. In addition, by allowing characters to raise skills in the areas of history, languages, religion, and other study-derived skills, players don't necessarily have to feel "min-maxer's guilt" over ignoring such skills during character creation, or sacrificing them when they level up or gain character points in order to keep their Broadsword skill climbing. If one can treat "book learning" as a special type of skill development that works outside of, but parallel to, the normal "go adventuring and get better at stuff" model, then players might be encouraged to treat books found in-game as something other than just GP in paper form that needs to be sold to a collector the next time the party hits a big enough town.
Of course, this begs the question - what do GMs do now about all these books that would have to be "written" in their campaign setting? I actually think this solves a lot of that problem. Often I found that if PCs found a book the first question would be "what do I learn if I read it?". Well, you can rattle off a few sentences about how the book is a history of the Argothian Empire or how it's a bestiary for the Upland Marshes, but you will get the inquisitive player who will actually want to know "everything". It's a little unrealistic to summarize a big fat dusty volume in one or two paragraphs - otherwise, that book would be a lot thinner! But if you used a skill-adjustment system with regards to some of these books, then the real content is mostly irrelevant: you have a time to read, perhaps a check against some sort of Intelligence or Research skill (representing the PC's ability to "learn" from reading), and if you're successful, you gain X% in the skill listed for the book. In fact, depending on how granular and stratified your system is, you could have a particular book give a small modifier to a general skill and a larger modifier to a more specific skill (Rolemaster or GURPS would be prime candidates for this option).
Of course, this idea isn't perfect. In more abstract and 'big-grain" systems the idea that one or even a handful of books will move you from "three dots" to "four dots" in a skill is a little silly. In this case, a helpful GM might rule that if you've got access to, or have read a book regarding that subject, you get a bonus to rolls in that skill with regards to that particular topic. Perhaps you'd list the book (or the "information" depending on whether you need the book at hand or just had to have read it) in your Equipment or Powers or Traits listing as a special circumstance modifier to certain knowledge rolls.
In the end, I think that regardless of how it gets implemented into the system, there are good reasons for making books and other forms of written (or heard) knowledge part of the rules. It wouldn't work for every system or every genre/playstyle, but I think there are some that would really benefit from the time and effort this would require.