Apple has at last released the Apple File System Reference, 143 pages and over 400 KB of detailed documentation about its new file system.

According to its revision history, this describes the data structures used for read-only access to APFS on unencrypted storage, but doesn’t apparently cover Fusion Drives, which hopefully will be detailed later.

I hope that the link given above doesn’t require a developer login, but it is the only one that I can find at present.