Ainslie's Complete Guide to Thoroughbred Racing is the classic guide to everything you need to know to pick winners.
Ainslie's Complete Guide to Thoroughbred Racing covers recent technological changes and the basics of breeding, conformation, calculating speed and pace, the factors of condition, class, and weight, rating jockeys, stables, and tracks, tote-board tips, and sixty tips for profitable betting.
This book is likely the bible of serious handicappers, but the style is ponderous, encyclopedic. But I will give Ainsle his due. The guy knew horses and horse racing like no one else.
For those that enjoy going to the thoroughbred races and betting on the ponies, this is a great start at making handicapping understandable. Goes through the entire process of evaluating horses and riders and races to give one a feel for which horse might have the best chance. While no handicapping system is perfect, this book at least gives you a feel for the information in the program. Ainslie also wrote a companion volume on harness racing.
If you're going to spend any time at the track, it's worth reading this book in order to understand how people attempt to win money betting on horses. Unfortunately, the house takes such a big cut that it's very hard to beat the odds, even if you are better than the other players (the other bettors).