The most comprehensive documentation for BitKeeper is in the BitKeeper Man Pages, where each command is described in detail.
We also have an overview of Nested repositories.
You can also read the BitKeeper/Nested Whitepaper for a motivation on the development of BitKeeper/Nested.
There is a wealth of information about BitKeeper in these design documents.
We have a good tutorial on merging with BitKeeper.
Tips & Tricks
Finally, there are a lot of tips and tricks we have compiled over the years.