This is information on what the various files in libdvdnav/src are for:

public API
----------

dvdnav.h		main public header
dvdnav_events.h		events returned by get_next_block()
dvd_types.h		some additional convenience types, mostly unused yet

dvd_reader.h		libdvdread API
ifo_types.h		IFO structures translated into C
ifo_read.h		reading and parsing of IFO structures
nav_types.h		NAV structures translated into C
nav_read.h		reading and parsing of NAV structures
nav_print.h		print NAV structures in a human readable form

client interaction
------------------

dvdnav.c		the big get_next_block() function and some basic housekeeping
dvdnav_internal.h	internal structure of libdvdnav library handle
navigation.c		basic Title/Part navigation
searching.c		PGC/PG navigation and seeking, position reporting
highlight.c		DVD menu highlight handling
settings.c		libdvdnav customizing
remap.[ch]		user defined reorganization of VOBU playback order

DVD virtual machine (subdirectory vm)
-------------------

vm.[ch]			the logic surrounding the DVD VM
decoder.[ch]		decodes and executes DVD VM commands

DVD reading (subdirectory dvdread, a modified copy of libdvdread)
-----------

dvd_reader.c		reading data from DVD sources
dvd_udf.[ch]		handles the UDF filesystem
dvd_input.[ch]		lowlevel input abstraction, libdvdcss glue code
md5.[ch]		md5 hashing to provide unique disc IDs

ifo_read.c		reading and parsing of IFO structures
nav_read.c		reading and parsing of NAV structures
nav_print.c		output NAV structures in a human readable form

read_cache.[ch]		caching

debugging and utilities
-----------------------

bswap.h			endianness helper macros
dvdread_internal.h	CHECK_VALUE() helper macro
vmcmd.[ch]		print DVD VM commands and registers in a human readable form