I adore “On Writing”, as well! It’s a fantastic read and really helped me get over one of my major blocks.
Also, you might want to check out Stephen Bonnycastle’s “In Search of Authority”. It’s not a book about writing, per se, but it’s an introduction to literary theory and it’s deeply fascinating. It goes through a lot of major schools of criticism and picks them apart for what’s more or less a basic college “lit 101” education contained in about 250 engaging pages. Really fantastic. I’ve been meaning to go and reread it, myself.