APFS technical documentation
Apple File System Developer Documentation
  
  Apple File System is a new, modern file system for iOS, macOS, tvOS, and watchOS. It is optimized for Flash/SSD storage and features strong encryption, copy-on-write metadata, space sharing, cloning for files and directories, snapshots, fast directory sizing, atomic safe-save primitives, and improved file system fundamentals.


See also Apple's File System Programming Guide.

Additionally, this blog APFS – A Backup Software Developer’s Perspective  makes mention of APFS already being reverse engineered.