Theory-like, Slansky's mandatory "The Theory of Poker". I've read several books so far and this one is on the top of my list. You'll hurt your finger if you like taking notes of everything though (I know I did). Lots of concepts and exceptions to keep track off, but definitely a good read. It will teach you both beginner and advanced concepts (pot
odds, implied odds, slowplaying, Heads-Up, bad beats, etc) and when and how you should do it. Some of those concepts (like effective odds, mostly) are a bit outdated, but it's no big deal. I recommend it to everyone. It gets kinda boring though. You don't get any side story, anything amusing, or the like. You get theory, and theory only. It's pretty much a college-like technical book on poker.
I also just started "Super System", so far, great read. Pretty casual and cool, not as technical as Slansky, not even near. Has some pretty good tips and gives you a whole new insight into poker, but I'm only just starting to read it so I can't really say much about it.
I finished reading "Poker For Dummies" in a rush yesterday since I borrowed it from a local library. If you are not a total beginner, or if you're not into video poker, or if you're not interested in other variants (if you are, this book only covers the basics so it won't get you very far there either) it's not worth the effort. It has a 1-page worth of tips, that's it. Makes for a good casual read, but uninteresting. It also only explores Limit games because the book is quite old and didn't go past (as far as I know) the 1st volume - for a good reason.
Keep in mind though, that both SuperSystem and Theory of Poker focus on all types of Poker. That's all I'm concerned with for now, so I can't give you any advice on tournament- or cash-focused books, not even HE- or Omaha-only ones!
Good luck at the tables!!
EDIT: I never manage to say "Sklansky" right but I'm far too lazy to correct that
