It may be useful for a single book to have an "XP limit" for a single pawn - that is, your colonist isn't going to keep reading the same, single book on construction for *literal years*, he's gonna finish it and need something else.

Additionally, this adds re-usability / value to lower-tier books: you don't want a pawn reading that "legendary" book until it'll be valuable (which makes thematic sense, too: somebody with *no* crafting experience isn't really going to get the full value out of that literally-legendary treatise on machining, y'know?); which means having lower-'quality' books still around for them to cut their teeth on has some value to your colony.