So far we do not provide a complete SOPE developer manual. Since the application server itself (NGObjWeb) was written with WebObjects compatibility in mind, you can reuse most of Apple's documentation on that topic.
Note that starting with WO 5.0 the Apple server was rewritten from scratch in Java. The WO 4.5.1 version is the last version which is supposed to be compatible to SOPE.
Also remember that sope-gdl1 is only roughly compatible with the EOF framework which was provided with WO 2.0 (EOF 1.1). For compatibility with a recent ObjC/EOF version, check out GDL 2.

Small snippets of documentation on various aspects of SOPE.