Some merchants only sell books and I would like to know if there is any real benefit in buying and "reading" them.

The ones about monsters will fill in the bestiary entries for monsters before you fight them. This can help so you know their tactics and what oils, signs, and bombs to use against them beforehand. It's not really necessary though. Once you fight a monster it gets added to your bestiary anyway. I generally only buy quest-related books.

The answer is quite simple: lore. Lore is what takes a mere game and makes it into an experience that you can believe in. If you've no interest in the lore of such a game you're just pounding meat for the sake of it.

I agree with Hickory. Books are always worth picking up - if nothing else then for the interesting read. Sometimes you can read passages that actually describe persons you have already met or explain conflicts and/or alliances in the universe. In short: If you like immersion (and who doesn't?) buy books. If the price is putting you off I can reveal that all books except for perhaps ~10-15 in the entire game (expansions included) can be found.