Really enjoying Writing Tools by Roy Clark.
It's the best book I've read on the craft of writing since “If You Want to Write by Brenda Ueland.
Ueland's book inspired me to write with my own voice. Clark's book helps me develop the skills of a writer.
$1.99 $8.99 on the Kindle app for Kindle, Mac, PC and iPad.
UPDATE: Somehow I got it for $1.99 on Kindle but the price has gone up to $8.99. Still worth it.
Plenty of hard copies available for as low as $5.