Caro's Book of Poker Tells written by the "Mad Scientist" of poker, Mike Caro, is one of the books that often has a legendary status conferred on it. It was the first, and is still just one of a handful, book that covered how to interpret various physical actions, called 'tells', at the poker table.
In the book, Caro presents a variety of physical motions and actions, some subtle, others over the top, and then interprets each. It also talks about people who are giving off fake tells, and how you can interpret those. In the end it all comes down to observation. Observe your opponent, think about what they are doing and what they ended up having, then store that information for the next time. Good players may give off tells and then do the exact opposite the next time.
The book contains graphics which appear to be from the original publishing of the book, as the people are all dressed in horrible 70's and 80's clothing. Many of the tells are so ridiculous you'd almost never see them. For purposes of the book, however, many of the pictures are exaggerated. Caro estimates the amount per hour you're able to make off the tell if you can pick it up, but these numbers are not accurate given today's games and how people are aware to tells now.
This book is not required if you play live, but it's a good introduction to the various types of things people will do. Better books exist now for picking up and interpreting tells, most notably articles and books written by Joe Navarro. However, this is is still worth reading if you're a beginner playing live.