I'm not sure how it works exactly, especially these days, but I believe it used to be something along the lines of this:

When you read a book that will give you a level, it will take you from whatever exp you have in that skill line to the very beginning of the next level. This makes books much nicer at higher levels, and more efficient when applied to skill lines that have only recently leveled up.

Again, I only believe this is how it at one point may have worked. I could very well be wrong. In fact, I probably am. Ignore everything I said. Unless I'm right, in which case, treat everything I said as gospel.