The next most important thing to experience in improving your game is reading books. Below I have a list of the books that I believe are the best resources out there.
I want to point out that I believe it is extremely important to read these in the right order. Too many players try to read the advanced books first (like I did) and screw it all up. You really need to learn to block and tackle before you run the zone blitz.
First book to read should always be David Sklansky's The Theory of Poker. This is widely recognised as a the definitive piece on the fundamentals of poker. I suggest that you read it slowly and ponder each point before continuing on.
Second book is Holdem Poker by David Sklansky. I am assuming this is the game you are most interested in. Even if it isnt, you can learn the most by mastering HE first and then tackling other games. This is because I think HE simplifies the # of possibilities and provides excellent training ground for the quantitative analyses needed in all games.
Third book is Gary Carson's The Complete Book of Holdem Poker.. This largely agrees with DS's book, but provides an alternative approach to some specific situations.
Next is Winning Low Limit Holdem by Lee Jones. He points out some specific strategy that is necessary given the very different psychology of the low limit game.
Now you can stop reading and get a couple years of experience. After a couple years, read Holdem Poker for Advanced Players by David Sklansky. This is a book that only helps you if you are playing against good player in higher limits.
Two tournament books out there are good. One is written by Sklansky and the other by Sylvester Suzuki. There are definitely some adjustments that are necessary in tournaments vs cash games. However this book also helps to address adjustments between various tournament formats.
Now, move on to no-limit poker and read Doyle Brunson's Super System. This book covers many games, and I suggest you read them all. The no-limit section is the Bible of poker.
Two other books which are excellent are Mike Caro's Book of Tells. The advice is spot-on, but since reading tells is only about 2% of your game, it comes later on the list than some other books.
Last, is the book that I most enjoyed. However, it rounds out the bottom of the list due the fact that the advice can only make sense after mastering the advice in the other books. That is Mike Caro's Fundamental Secrets of Winning Poker.
Other books I have read (or read about) and enjoyed are Mason Malmuth's Poker Essays, Poker Nation, and Poker for Dummies by Lou Kreiger, all of which are educational.
I have only read a couple bad books, and I don't remember any of the titles.
Random thoughts from a lawyer, an accountant, a commodities trader, an ex-Marine and a WSOP Main Event money finisher that don't know as much as they wish they did...