Soaring Flight Manual - private/commercial - Used
by Jeppesen Sanderson and the Soaring Society of America
This book is no longer in print.  At the bottom there are links to used copies.
Paperback, about 200 pages
Dimensions: 6.0 x 9.0 x 0.63 inches (153 x 229 x 16 mm), Copyright: 1978, 1979, 1980, 1984

This was the standard training manual in the USA when I started soaring in 1990.  It does a good job of explaining the principles of soaring and flight skill necessary for flying sailplanes.  It has been replaced by the Glider Flying Handbook.

Search for used copies at: alibris or amazon.com or Barnes&Noble or ebay