A list of information sources:
First of all: GitHub EGroupware Wiki
Here you can find many different Information.
Code Corner
These documents are outdated, but not useless.
Special thanks to Nicklas (Nilpferdschaf) for his work.
Code-corner EGroupware-Wiki
Code-Corner Nilpferdschaf
The most documentation is inserted in the source code. Here are some (important) points:
Topics:
VFS/SMB
Port to et2
The documentation to port etemplete applications to etemplate2: