You are still in Act 1? I know, you are always broke ;D Important books are about plants and monsters. All others about lore, etc. you can neglect so far. btw... if you challenge a guest (drinker) in the inn and defeat him he gives you an important book... unluckily I don't remember the title.
Ditto to above. Lore and history books really aren't too important at this point, though they do help to fill a lot of back-story and do have some interesting tidbits of info.But the books on herbs, plants, and monsters can be invaluable. One thing though; before you blow all of your orens on these books, you should definitely get around and talk to people as much as possible before doing so because a lot of times, you can pick up journal entries that way. You just have to keep track of what you've learned before buying; there's nothing more frustrating than wasting money on a book and not receiving any new entries because you've already learned everything in that book...
I think it's better you save your orens if you are in act1. Some infos about plants and monsters you can have by talking to peasants especially the old women. They ask you some food. the same for the next chapters. In act2 you'll find a very useful librarian and you will have enough orens to buy.So be patientenjoy
Already in Act 1? I don't rememeber such a book (of disappointment). Later in the game you can bear it cos you have enough money. Well, 400 Orens at the beginning is much but I never got stuck because I bought an unbearable expensive book... I missed my chance to get for free the formula from Abigail for spectre oil >..< but even so I made to Act 2.
I dont think I bought a single book in Chapter 1?Talk to old ladies and women, offer food as a gift where the option arrives, and wait until chapter two.Personally, I`d say buy books on monsters first (covering the monsters you`ll meet in Ch.2) ... th reason being, that once youve identified those monsters by reading the book and getting the bestiary entry, you are then able to fully "skin" them and get the meat, pelts, or ingredients their corpses give up.Closely followed in priority for me was the Herbology books.History/Lore books should be last on your buying list.You can actually findmost ( all?) books dotted around the game in houses, chests, drinking games...etc... so you could get through without buying any.... but I wouldnt recommend it.I have a very simple answer/solution to the "no money for books" problem...Play poker, with everyone who plays, every day, and take all their money.Get used to doing that every day in the game, and by chapter 3 you`ll have pockets full of cash, and the brain capacityof a professor from all the b ooks youve read and resold 
*signed* on my first gameplay I didn't use dice, couldn't take a liking in it. Sure I had much many as well... but much later in the game. As soon as you can take a bath in Orens and have no idea how to spend it I recommand a lore bout about "Hexlinge" (if it has the same expression as in German). It's a scholary book about "wichters and other creatures"... good fun to read ;D
you can find a lot of books by just looking around, searching trunks and cabinets.if you're low on funds, it's probably advisable to wait with a purchase until you're positive that you need it, for example, if you've picked up a contract on a certain monster. then you should go looking for the book that will allow you to complete the contract.later on in the game you get enough money to just buy books whenever you please.