Dan Harrington is the 1995 World Champion. In his books, he shares his knowledge and teaches you a very straightforward but also very successful way to play. I think this book is the first you should read if you begin playing No-Limit Hold Em (assuming you know the rules). Harrington's books speak about No Limit Hold Em tournaments, but the first volume explains the basics and is very helpful even if you just want to play cash games. It is not a simple introduction to the game. This book teaches you how to play and also how to THINK.

It explains the most important notions:
- drawing odds
- pot odds
- implied odds
- bet sizing
- psychology
- most important concepts (continuation bet, GAP concept, squeeze play...) to play more than only your hole cards
- how to play against the different player styles (tight, loose, passive, aggressive...) by adjusting your game.

There is a quiz test after each chapter which really helps to understand the concepts. Harrington on Hold'Em Vol.1 is an excellent book with great material, and a great first book if you want to begin to understand the theory and the key concepts of No Limit Hold Em. It is a must-read for any player who wants to improve their game.