Tundra Developer Documentation
Introduction and Architectural overview
There is an introductory article from the early days of Tundra:
Alatalo Toni (2011). An Entity-Component Model for Extensible Virtual Worlds. Internet Computing, IEEE. Sept.-Oct. 2011. Volume: 15 Issue:5 pp. 30 – 37 http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=5928309
That article was and some other article drafts are worked on in http://github.com/realXtend/doc .
End-user documentation and development tutorials
Meshmoon has good basic both end user and developer documentation for Tundra SDK now — huge thanks to them for the work and having the docs on-line for us all: http://doc.meshmoon.com/ .
The Meshmoon specific parts are marked there so you should be able to know what applies to basic Tundra and realXtend tech and what is Meshmoon hosting specific.