I dare say books exist and other members may help, but why not just start?
If you play Gershwin, take one of those pieces, go by the chord sequences - play the
chords in the inversions used in the printed music, then freak around with the tune over the top. Choose a chord or key and improvise anything over it. Keep it simple at first.
Listen to lots of jazz on the radio/CDs.
I'm in your same situation. It's a fair bit about 'letting go'. When you start, that's difficult because you have to find your way around the keyboard without printed music. Persist. It won't happen over night but it does happen. I reckon you need to develop a good ear - practice goes a long way to that so your hands fall on what you want to play without too much thought of the mechanics of playing. I'm not quite in that league yet! but getting there.

R