NTFS is proprietary. I don't think a full set of official and definitive documentation for the theory of operations and on-disk structures exist, outside of licensed and non-disclosable Microsoft documents, which are not available to regular end-users. There is a SDK that Microsoft makes available for partners who want to built file systems for Windows, and that requires interesting licensing. Using that SDK requires a significant amount of work; I remember several colleagues having to travel to Washington State for many days of meeting. I don't think even with that SDK one gets access to the full NTFS internals, only to the interfaces that allow partners to develop their own file systems for Windows OSes (not to read and write NTFS).
There is some information that is available from Microsoft about how NTFS works (seen from the outside), as part of the API for other file systems. This explains things like timestamp formats, reparse points, hard links (they are different from Unix hard links), and so on. Some of that knowledge ends up being visible in the documentation for the CIFS protocol.
There is an enormous amount of information about NTFS available from the various projects that have written implementations that can read (and sometimes even attempt to write). Just go to the NTFS wikipedia page and start clicking on links. Since that information was usually gathered from black-box analysis, its correctness is doubtful.
Apple HFSplus: I know of no documentation for that, but I haven't had any need to look. Since HFSplus is directly descended from the standard BSD UFS and FFS file systems, it should be relatively easy to study this by observation.
Never mind, Obsigna found real Apple documentation of the on-disk format. Amazing, thank you.
