How to start with BookKeeper depends on who you are...
Developers who are new to BookKeeper should start with the BookKeeper Tutorial. The tutorial shows you how to build a basic distributed system using BookKeeper. Turning ledgers into logs gives a briefer description of the principles behind the logs used in the tutorial.
Once familiar with the basic concepts, developers can consult the BookKeeper Java API documentation.
Administrators will be more interested in the BookKeeper Admin guide. It describes the steps involved in setting up and maintaining a cluster. The available configuration parameters can be found here. An important aspect of BookKeeper is how it deals with the failure of storage nodes. This is covered in Bookie Recovery.
Contributor documentation is less organized, BookKeeper Internals is a good place to start. From there you can check out our wiki and ask questions on our mailing lists or IRC.
