The main jump is in the 12.5 to 15.0 step. From 15.0 to 15.5 and 15.7 the changes aren't so dramatic.
The Sybase portal has some interesting white papers. The official "What's new..." manuals are also interesting, but you might prefer something more summarized. For instance, "New Features Summary. Adaptive Server Enterprise 15.7" has 150 pages.