Watercolor Quilts; Pat Maixner Magaret, Donna Ingram Slusser (Contributor), Barbara Weiland (Editor). This is the classic book on watercolor quilts. It has complete instructions, and many beautiful examples.
Watercolor Impressions; Pat Maxiner Magaret, Donna Ingram Slusser (Contributor). This is the followup book to Watercolor Quilts -- it's a compendium of pictures of watercolor quilts. Don't look for instructions from this one, but it's wonderful for inspiration!
Impressionist Quilts; Gai Perry. This is more art oriented than Watercolor Quilts -- it's an excellent next step. It focuses on fabric and color selection, as well as advanced techniques to create an impressionist masterpiece.
Impressionist Palette : Quilt Color & Design; Gai Perry. The followup book to Impressionist Quilts, this book is great on its own merits. It goes more in depth into the elements of design and color selection, and introduces new techniques and patterns.
Strip-Pieced Watercolor Magic : A Faster, New Approach to Creating 30 Watercolor Quilts; Deanna Spingola. A quicker way of making watercolor quilts, this book demonstrates how to use strip-piecing (sewing strips together, and cutting them into rows) to create beautiful watercolor quilts in much less time.
More Strip-Pieced Watercolor Magic : New Designs and Techniques for 30 Watercolor Quilts; Deanna Spingola. This is the followup to her first book, with some new techniques and pattern to add complexity and interest to your watercolor quilts.
Quick Watercolor Quilts : The Fuse, Fold, and Stitch Method; Dina Pappas. A newer take on watercolor quilts, using just a few fabrics, and fusible interfacing. This makes for very fast quilt design and assembly, with some impressive results.