All the FITSIO documentation and source code are freely available electronically via anonymous ftp from the legacy.gsfc.nasa.gov site in the /software/fitsio directory. FITSIO programmers should be sure to obtain a copy of the FITSIO User's Guide (in the files fitsio.doc or fitsio.tex or fitsio.ps) which gives a detailed description of the FITSIO user interface.
The FITSIO documentation assumes that the user has some familiarity with the definition and structure of FITS files. The NASA/GSFC Science Office of Standards and Technology (NOST) can provide a wide range of information and help regarding FITS formats. In particular, the NOST maintains 2 very useful documents: `A User's Guide for FITS' and the `NOST FITS Standard'. The former gives an introduction to the history and structure of FITS files, while the latter provides the authoritative definition of FITS formats. The NOST can be contacted via the internet at email@example.com, or by telephone at (301) 286-3575. They maintain an anonymous ftp site at nssdca.gsfc.nasa.gov which contains the above documents as well as other FITS software and information.