OPENSER Devel Guide has been published as html today. It is comprehensive tutorial, approaching the important components of OPENSER: memory manager, locking system, pseudo-variables, transformations, statistics, configuration file, db and timer api, module interface and so on.
The document is intended to give the knowledge for those willing to develop extensions for OPENSER and to show its internal structure. The tutorial is available at:
http://www.asipto.com/pub/openser-devel-guide/