There's no "one size fits all" answer here. Books may not be relevant for everyone anymore - although I have several hundred in my library inc +100 chess books.
If you have the time and patience to read chessbooks you can learn a lot IMHO from books intended to instruct such as "My System" "How to Play Like a GM" "Common Sense In Chess" etc etc
You can also learn a lot from the Tactics Trainer, Chess Mentor, Videos etc all available here. It all boils down to which study method(s) you prefer and which ones seem to work the best for you
In a recent chessbook review, IM Bryan Smith said that he does not play through every game in his chessbooks. He sits and reads through the book, like a novel or something.
What about you guys? how do you treat your chessbooks? Do you think that chessbooks are still relevant / useful in todays realm of Fritz and chessbase?
Furthermore, what is the most effective way to use a chessbook?