The LambdaMOO Programmers Manual, the programming core that lilyCore uses.
The primary author of lilyCore, Harq al-Ada, has been providing classes on both LambdaMOO programming and on programming for lilyCore specifically, including a description of the internal objects of the core.
Programmers of lily clients have some internals outlined.
Please be sure to update any help files that are affected by your programming changes.