What If? (Writing Exercises for Fiction Writers) is a wonderful book by Anne Bernays and Pamela Painter. The ideal companion to Bird by Bird, it contains 83 exercises and discussion pieces designed to improve your fiction writing.
Unlike many books in the genre, which seem to have been written by hacks wanting to make a quick buck off hopeful authors, each chapter is genuinely helpful. It's often incorporated into university creative writing courses.
It's published by Harper Collins, and costs around $13. The ISBN is 0-06-272006-6.