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 at the Haiku web site.
If you prefer to see how it works in code, please have a look at Haiku's BFS implementation.
If you are interested in the Haiku file system API, please refer to its documentation as part of the Haiku book.