In addition to those already mentioned, here are a few I've enjoyed:

Sierra Sierra by John Joss
Exploring the Monster by Robert F. Whelan
Soaring Beyond the Clouds by Bertha Ryan
Soaring by Peter Dixon
Stalking the Mountain Wave by Ursula Weis

Another which was extremely educational and I cannot remember the title or find it on my shelf is the anthology of the Soaring Symposium lectures.