Though I have bought lots of books over the years, I go back to a couple of goodies to get new inspiration time and time again.
Try "Calico and Beyond: The Use of Patterned Fabric in Quilts"
by Roberta Horton . The focus is on using more challenging fabric types and colors. It really pushed me beyond straight traditional quilts (which I love and feel deeply connected to) without leaving traditional blocks behind. The book also introduces exercises for Asian and African-American style quilts.
If I had to only keep a dozen books this would be in my top 3!