David Barrett's book will get you started to understanding the basics of how to improvise. It's all about learning from the masters, knowing the forms, and some basic theory. As he quotes from one of our great players today, "It's highway robbery!" You don't have to make it up from scratch, you just learn how to repackage it in a way that fits your feeling for the blues and how you're interacting with other players or the jam tracks he provides.
I thought I couldn't improvise and now I do. I'm not great at it yet, but I'm continually improving and, just as I had faith I would get here, I know I'll go further as well. After doing several of David Barrett's books for finding your way around the harmonica, you can get right into this one and follow it up with his books on Improvising, where he gets more deeply into it and accompaniment.
I cannot imagine how there could be any other books like these. Want to understand the mysteries of improvising. This is the place to start!
Annamarie Jackson