I agree with @CJ. Brad Free's "Handicapping 101" is a winner for me. I have it both in hardcover and Kindle formats.
I personallly do not like and can't tolerate any book that reads like a text book that you were required to read in college. Many handicapping books do read like a collegiate text book to me, and I can't get through a chapter without falling asleep or becoming bored out of my mind. To me, Free's book is more of a "Mentor-Mode" book and reads like someone is trying to teach you something around a campfire, rather than by filling your head with thousands of back-story building blocks in a classroom setting so they can give you a test later and see if you absorbed enough to pass.
My favorite Introductory to Intermediate handicapping book (and yes, I get a lot of flack for saying this) is "Betting on Horse Racing for Dummies". I know, the title suggests that it's not worth cracking the cover, but the author is Richard Eng (former Turf Editor and Handicapper for the Las Vegas Review-Journal) and it's an interesting and slightly entertaining journey into handicapping. I wish I had read it when I first started.
LINK to Amazon