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.

Getting Into It does a fine job of demystifying improvisation. It's an eight-note book, so probably best suited for the late beginner/early intermediate player.

IMO the three slender book/CD packages in Sowash's That's Jazz series will help teachers to help their students realize how natural and accessible improvising is.

The review includes a link to reviews on the author's site, including one in the April/May 2008 American Music Teacher.

Hope you'll have the time to read my review. Here's a link.