I have an ever-growing collection of instructional art books. If it's a great book, I tend to refer back to it again and again. They can be a great starting point when I'm in a creative rut. Here are a few titles I have recently read and enjoyed.

The Art of Expressive Collage: Techniques for Creating with Paper and Glue by Crystal Neubauer.
Art, Inc.: The Essential Guide for Building Your Career as an Artist by Lisa Congdon
Steal Like an Artist: 10 Things Nobody Told You About Being Creative by Austin Kleon
Wabi Sabi Painting with Cold Wax: Adding Body, Texture and Transparency to Your Art by Serena Barton