I borrowed a really nice book in the library some time ago which I was really impressed by. It was like an ECO but made really simple and there were short and simple, easy to understand explainations on each opening. Also like the ECO giving some lines that are considered critical and with an evaluation in the end.
Unfortunately I don't remember the name of the book, I'm definitely gonna have to borrow it again! ^^
Haha 
when you remember, feel free to share the name here 
Tnx firepower, I'll consider your recommendation
The book Conzipe speaks of might be Van Der Sterren's "FCO: Fundamental Chess Openings". Highly recommended as an overview of the openings, well written and a pleasure to read.
Interesting and not too pricey
Many, many moons ago I used I.A. Horowitz's "Chess Openings: Theory and Practice". I don't really know what other people think of it but I still like it. It has more explanation and is less dry than an ECO but still has examples of main lines and variations. I'm sure it's way obsolete if you're interested in current opening theory but it could be very useful if you want the basic theory of many openings.
It also seems very affordable on Amazon if you look at the used copies.
http://www.amazon.com/Chess-Openings-Practice-I-Horowitz/dp/0671205536
Hello! Can someone recommend me a great book about openings? Not a book about one specific opening, but a book about all openings and opening principles! tnx for the recommendations, cheers!