Here is a list of all documented files with brief descriptions:

fsliolib
dbh.h
fslio.c	Main collection of FSL i/o routines, written by Mark Jenkinson, FMRIB
fslio.h	Data structures for using the fslio API. Written by Mark Jenkinson, FMRIB
niftilib
nifti1.h	Official definition of the nifti1 header. Written by Bob Cox, SSCC, NIMH
nifti1_io.c	Main collection of nifti1 i/o routines
nifti1_io.h	Data structures for using nifti1_io API
znzlib
znzlib.c	Low level i/o interface to compressed and noncompressed files. Written by Mark Jenkinson, FMRIB
znzlib.h