I want to uniderstand in more detail how filesystems work - not how to use them, but how they are laid out on disk. Ideally, to such an extent that I could inspect a disk device with a hex editor and understand what I see - in the past I've got as far as 'inodes', but in most cases it goes a bit vague after that, and I can understand why, since there are so many different filesystems.
This is not for any purpose other than curiosity; and who knows, it might become useful. So, is there any good documentation or discussion of these subjects somewhere? Or is it back to kernel sources?