Probably the best resource for a description of the inner workings of BFS is the book "Practical File System Design with the Be File System" written by Dominic Giampaolo and published by Morgan Kaufmann Publishers. Although that book is out of print, it's available as a free download from Dominic's web site.
If you prefer to see how it works in code, please have a look at Haiku's BFS implementation - you can find that in the SVN repository under src/add-ons/kernel/file_systems/bfs/. This code is not mature yet, but is considered to be fairly stable.
If you are interested in the Haiku file system API, please refer to its documentation as part of the Haiku book.