Many moons ago, when I was a newbee I purchased a book by Andy Beyer...it has been so long ago I don't remember the exact name, something like "Winning at the Track."
Gave some good background on horseracing, some good horse race betting stories and he postulated his 'speed figures' concept. Which as we all know are a main staple of the Daily Racing Form now.
There are many, many ideas of how to handicap, and all are good to a certain extent. As you go along, learn what works best for you and go with that.