Most Contiki developers hang out on
contiki-developers mailing list. The list is
a friendly place where good questions asked the right way
typically receive constructive answers.
The Contiki community wiki contains community-provided information about Contiki.
The Contiki source code contains Doxygen mark-up that is used to automatically generate Doxygen documentation.
These two books cover the Internet of Things technologies:
By JP Vasseur, Cisco, and Adam Dunkels, Thingsquare.
By Zach Shelby, Sensinode, and Carsten Bormann, TZI.
Many of the mechanisms behind Contiki have been published in the computer science literature through the papers below. These papers provide technical details and reasoning behind many of the Contiki mechanisms.