Super/System 

This book is written in 1979 by Doyle Brunson. When Brunson played Texas Hold'em he took a deep dive into the strategies who used to make big money. There is also Super System 2 (2004). 

Harrington On Hold ‘Em

This book is written in 2004 by Dan Harrington. This turned out to be Volume 1 of a three-part series; all three are worth checking out, but the original book on strategic play in tournaments should be first on your list. Harrington was the 1995 World Series of Poker Main Event champion and a former WPT champ as well.

The Mathematics Of Poker 

This book is written in 2006 by Bill Chen and Jerrod Ankenman. If you’re not scared of a little math, this book will teach you more about poker than you ever thought possible. The authors use mathematical principles and toy games to break Hold’em down and generate useful strategies that pretty much all of today’s top players hang their hats on.

The Theory Of Poker 

This book is written in 1999 by David Sklansky. Sklansky just might be the world’s foremost authority on poker, and gambling in general. He’s written several books under the Two Plus Two Publishing aegis, but this one’s the most important. It covers the theoretical foundations of both draw poker and flop games – and the theory holds up to this day.

Caro’s Book Of Poker Tells 

This book is written in 2003 by Mike Caro. Mike Caro was one of several players who collaborated with Brunson on Super/System. Caro’s own highly influential book looks at the behavior of poker players at the table and how to use it to your advantage. There’s a lot more to it than just “weak means strong” and “strong means weak.”

Poker Books are as important as ever. Even though there's a lot of content out there, poker books are still something to improve your game by.