Btrfs - What to Read and other Questions

Hey Guys,
I am reading through the btrfs code and am wondering what files/docs I need to read in order to start helping out with btrfs. In addition when you state size and offset in the page explaining On Disk Format, I am wondering what you mean by offset and size it the tables expanding definitions of certain key values to the disk layout of btrfs , I have an idea but just want to make sure I am understanding it correctly through :).
Regards and Thanks,
Nick