The super system by dole brunson. Its an old book but its really good at cash games. There are a lot of tips and theory that never go old and I really enjoyed the formate of the book. I recommend it and you should go about reading and practicing it. Enjoy.
Super System is a classic - no one can deny that. But each time someone recommends it as the essential work I have to question what else they have read on the subject. Since SS came out 35 years ago an incredible amount of poker books have been published - multiplying (not just adding to) the number available in 1979 when it came out. Not only has the game evolved and changed, so have teaching methods and writing styles on the subject. And while I am not going to say SS is not worth a look I (and I think anyone else who has a poker library that covers more than one shelf will agree with this...), I can say that since SS was published several books. training sites, blogs, etc. have vaulted above and beyond what Doyle put forth 35 years ago. In other words, several authors (and groups of authors) have just done it better.
Also be aware that SS is NOT about Texas Hold Em - at least not until about page 330. Up until that point it is focused on Draw, 7-Card Stud, Lowball, High-Low, Razz, etc. And not much, if any, in the way of tournament strategies. So if you are only playing Hold'Em, this is probably not the book to start out with.
I have no problem with someone saying it is a must read, as exposure to these other games and their strategies will help stretch your own strategic muscles in Hold Em or whatever. Don't be fooled though. This is NOT (though it was at one time) the ultimate strategy book, ESPECIALLY if you are only playing HoldEm.
Also, the answer to this question is rooted in what stakes you are playing for. Strategies radically change as the blinds go up. What you can do (and what will be done to you) in a .10 BB game compared to a $10 BB game are very different. Not to mention live vs online.
Without that information it's just a guess and most people are going to offer up the book that helped them in their stakes and situation, which is also not usually mentioned. Point is a lot of these suggestions can be very misleading to you and could end up hurting your
bankroll. So the more specific you are about the help you are looking for the more specific the help you will get. And being specific is pretty damn important when playing poker.