Collage Lab by Bee Shay offers beautiful technique ideas on the media of collage.
Print & Stamp Lab by Traci Bunkers offers fun ideas of everyday items that can be used to create interesting stamps. Her ideas on layering of shapes and stamps are particularly interesting.
Art Lab for Kids by Susan Schwake is one of my favorite all-around mixed media inspiration books to use with students. There are great ideas for enrichment and one-day lessons.
Drawing Comics Lab by Robyn Chapman, is definitely for older audiences in subject and presentation. That being said, there are some really great ideas on how to get started creating comics, how to package them for sale, and how to get started on making a profit from your own comics.