Hard to choose just one, but I find the ones I use most often as a reference are QUILTS! QUILTS! QUILTS! and QUILT! QUILTS! AND MORE QUILTS! by Diana McClun and Laura Nownes. All of the other books I thumb through for inspiration. I love them all. Really like 101 FABULOUS ROTARY-CUT QUILTS by Judy Hopkins and Nancy J Martin. Then Mary Ellen Hopkins' IT'S OKAY IF YOU SIT ON MY QUILT BOOK is great for reference.