I just posted a review of Bradley Sowash's introduction to improvising on the Pianist, Non-Classical forum. (Sowash is the co-author of The Next Step, which I reviewed previously.) The thread title is For The Beginning Improviser - A Review. Here's a link.

Getting Into It does a fne job of demystifying improvisation. It's an eight-note book, so probably best suited for the late beginner/early intermediate player. Based on some of the work I've heard on the Piano Bars, some of the folks here are ready for this one. And there's no law that says that you can't play the practice tunes at half tempo.

The review includes a link to reviews on the author's site.