Is there value in reading books to learn more about poker? Are the $25 you spend to buy that book, and the hours it takes you to devour its contents, actually worth it? The standard, off-the-shelf answer would be a "yes," without a doubt. I'll be a little less conforming to the standard and say "it depends on what you want."
There's no doubt in my mind - none whatsoever - that if you have not yet read an actual book on poker strategy, the quality of your game will benefit greatly from picking up one of the books I recommend. Poker, however, is not always just about the money, despite (ironically) that most authors will tell you that it is. You've chosen to play poker, and you may have done so for a variety of reasons, none of which may be "because I want to be the best I can be at this game," or "I want to make as much money as I can off of it." The fact of the matter is that if you're playing poker solely for the enjoyment, or for the sake of killing time, or for the thrill of gambling, reading books may be something you're not at all interested in, and thus should not be spending your time on. If "making some spare cash" is anywhere on your list of goals for your poker game, however, you'd be doing yourself a disservice not to study some of the texts available. But it's your choice.
You may be the kind of player who's self-taught (or close to it), who's already a winner - or at least have been so far. What could a book bring you? The short answer is "lots of money." You have to realize that a lot of the players at your tables have read books, and some have probably done some serious studying. The fact that they are armed with strategies and plays that you're not even aware of means that their edge on you is huge. If you learn what they know, you'll not only reduce the edge they have (or even overtake it) but also gain that same huge edge against the non-educated players at the tables. Chances are you'll become aware of large mistakes you're constantly making.
Do you understand the concept of pot odds? Of implied odds? Of reverse implied odds? Of when you want to just call on the flop in hold'em because you want to be able to protect your hand on the turn? Do you realize the difference between raising for value with a speculative hand, and when you should do it as a semibluff?
You should be aware that just reading a book on poker strategy will not in itself change your game. You need to be prepared to consciously think about your game, and practise. The books that are recommended in this section should all be studied in a fashion where you read the book, and then attempt to apply the principles - one at a time, preferably - to your actual game. This is not an easy task. You will likely - not to say certainly - read each of the books several times, to make the most use of them. And when you've read a chapter, you should think it through, to find a way that you can practically apply the lesson it teaches you to your own playbook. To pull this off, you need to sit down and play hands, and for every hand you're dealt make a conscious decision with every action you take, and try to find the corresponding situation in the texts you've read. If you do this, your learning and your improvement will be monumental - this I can guarantee you.
But, and I want to make this perfectly clear, serious studying is not for everyone. If you're a casual player, I still think you would be well off to read a book or two (and I will give you recommendations of which ones to pick up), but you're probably not going to want to spend hours - more like weeks or months - on studying them and practising how to apply the concepts to your own game. Read the "For The Casual"-recommendations and find out how you get the most out of books without digging too deep.
If you want to be serious about your game, learning as much as you can to increase your edge or your enjoyment out of the tactical nuances of the game of poker, I have prepared a list of recommended reading as well.
In any given session, you are likely to come across dozens of interesting situations, even if you don't necessarily realize it. Maybe you made the right move, but for the wrong reason. Maybe you made the wrong move, but your thinking was good. To truly develop as a poker player, you must be able to review your own decisions with a critical and analytical mindset. I've included a specific chapter on this, because it's that step that can ultimately take you from being a player who knows the theory, but fails to apply it, to one who can identify and relate to every concept in the books you've read, and make full use of them.