wesp5: It was kind of hidden by moving the books outside of the shops. Still people knowing about the exploit could find a book, sell it to a vendor and then buy it back multiple times! I have lately stopped that by making the books not sellable...
lordhoff: I thought the book exploit could only happen once, ie, you buy it, read it, sell it, buy it read it sell it then it's no longer even available (or am I thinking of something else - my thinking is the books sold in the pawn shop.)? I've always assumed that it was intended to be that way by the developers, especially since the books only worked for a very limited range (I think it won't work above level 3?).
All books have a minimum and maximum level that your skill has to be at in order to be able to use it. Apart from the top-level books (which can only be used to raise your skill up to level 5, ie you can only use these once), all other books may be used in a 2-level range (0-1, and 2-3).
I don't think they intended this exploit - because let's face it, it is an exploit and it defies both real-world logic and general in-game logic.
The way books level does make some kind of sense - low-level books only help when your level is low, but are useless for higher levels. Intermediate books require some knowledge before they're useful, but too much and you already know everything they could teach you.
If I had to guess, I'd say that the level "ranges" for most books (as opposed to making them all only work to upgrade from/to a specific level) is to increase their utility, i.e. it allows players to still make use of them if they've leveled their skill a little bit already, instead of being paperweights for these people.
Unfortunately, once players figure out that they work across TWO levels, then from that point on one makes sure to have leveled to the highest point possible prior to using them, due to the XP point savings. Although this is only true of the mid-level books - the top-level ones only have one level they work on, and the lower-level ones have the same XP cost either way. So, the whole book thing is likely a result of half-assed tweaking and compromises.,..