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

event2
buffer.h	Functions for buffering data for network sending or receiving
buffer_compat.h	Obsolete and deprecated versions of the functions in buffer.h: provided only for backward compatibility
bufferevent.h	Functions for buffering data for network sending or receiving
bufferevent_compat.h
bufferevent_ssl.h	OpenSSL support for bufferevents
dns.h	Welcome, gentle reader
dns_compat.h	Potentially non-threadsafe versions of the functions in dns.h: provided only for backwards compatibility
event.h	Core functions for waiting for and receiving events, and using event bases
event_compat.h	Potentially non-threadsafe versions of the functions in event.h: provided only for backwards compatibility
http.h	Basic support for HTTP serving
http_compat.h	Potentially non-threadsafe versions of the functions in http.h: provided only for backwards compatibility
listener.h
rpc.h	This header files provides basic support for an RPC server and client
rpc_compat.h	Deprecated versions of the functions in rpc.h: provided only for backwards compatibility
tag.h	Helper functions for reading and writing tagged data onto buffers
tag_compat.h	Obsolete/deprecated functions from tag.h; provided only for backwards compatibility
thread.h	Functions for multi-threaded applications using Libevent
util.h	Common convenience functions for cross-platform portability and related socket manipulations