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

lib
bitmap.h	Bit map API
dkhash.h	Dual-key hash functions for Nagios
fanout.h	Simple fanout table implementation
iobroker.h	I/O broker library function declarations
iocache.h	I/O cache function declarations
kvvec.h	Key/value vector library function and type declarations
libnagios.h	Include this for all public parts of libnagios to be accessible
lnag-utils.h	Libnagios helper and compatibility macros that lack a "real" home
nsock.h	Nagios socket helper library
nspath.h	Path handling functions
nsutils.h	Non-Standard (or Nagios) utility functions and macros
nwrite.h	Functions that properly handle incomplete write()'s
prqueue.h	Priority Queue function declarations
runcmd.h	Runcmd library function declarations
skiplist.h	Skiplist library functions
snprintf.h
squeue.h	Scheduling queue function declarations
t-utils.h