So most RPG's with merchants in them, the amount of credit given for particular items, vary somewhat between them. Witcher 3 is no exception – some will give a lot for weapons e.g. and some will give not 1/10 of that. The same goes for every other type of item. If you are a loot whore like me, you probably have a whole lot of books, and they are probably annoying as hell because they fill up your "usable" inventory slot, making it difficult to find the right potion, oil , etc., in a tight fight. So the hint here? Instead of just selling your books for 1 crown at some random merchant, go to the square in Novigrad and sell them to the book merchant there. He will give the full price for them, and there is some 500-600 in his starting capital. He wont be there all day though, so if he isn't there, just meditate until he appears.
UPDATE
So a lot of you asked who the merchant is. Here he is. He is found around 7pm ingame as you can see. But he isn't there all the time.