Try these:
Blair, Seabury. Backcountry Ski! Washington. Sasquatch Books, 1998.
Mostly shorter/winter season tours, but I've used it a lot. A bit of non-Cascade stuff too. Repetitive but good maps.
Burgdorfer, Rainer. 100 Classic Backcountry Ski and Snowboard Routes in Washington. The Mountaineers, 1999.
The standard, with lots of long hard tours. Maps hard to read.
Van Tilburg, Christopher. Backcountry Ski! Oregon. Sasquatch, 2001.
Good also, tho I've only used this a few times. Focuses on volcanoes, w/ snowboarding emphasized. Lots on Hood, a bit on Wallowas, some S. WA tours.