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

guacamole
argv-constants.h	Constants related to automatic handling of received "argv" instructions
argv-fntypes.h	Function type definitions related to automatic handling of received "argv" instructions
argv.h	Convenience functions for processing parameter values that are submitted dynamically using "argv" instructions
assert.h
audio-fntypes.h	Function type definitions related to simple streaming audio
audio-types.h	Type definitions related to simple streaming audio
audio.h	Provides functions and structures used for providing simple streaming audio
client-constants.h	Constants related to the Guacamole client structure, guac_client
client-fntypes.h	Function type definitions related to the Guacamole client structure, guac_client
client-types.h	Type definitions related to the Guacamole client structure, guac_client
client.h	Functions and structure contents for the Guacamole proxy client
display-constants.h	Provides constants related to the abstract display implementation (guac_display)
display-types.h	Provides type definitions related to the abstract display implementation (guac_display)
display.h	Provides an abstract display implementation (guac_display), which handles optimization automatically
error-types.h	Type definitions related to return values and errors
error.h	Provides functions and structures required for handling return values and errors
fifo-constants.h	Provides constants for the abstract FIFO implementation (guac_fifo)
fifo-types.h	Provides type definitions for the abstract FIFO implementation (guac_fifo)
fifo.h	Provides an abstract FIFO implementation (guac_fifo), which can support arbitrary element sizes and storage
fips.h
flag-types.h
flag.h
hash.h	Provides functions and structures for producing likely-to-be-unique hash values for images
layer-types.h	Type definitions related to Guacamole layers
layer.h	Provides functions and structures required for allocating and using layers
mem.h	Provides convenience macros/functions for performing arithmetic on size_t values and for allocating memory, particularly memory related to images, audio, etc
object-types.h	Type definitions related to Guacamole protocol objects
object.h	Provides functions and structures required for allocating and using objects
parser-constants.h	Constants related to the Guacamole protocol parser
parser-types.h	Type definitions related to parsing the Guacamole protocol
parser.h	Provides functions and structures for parsing the Guacamole protocol
plugin-constants.h	Constants related to client plugins
plugin.h	Provides functions and structures required for handling a client plugin
pool-types.h	Type definitions related to the guac_pool pool of unique integers
pool.h	Provides functions and structures for maintaining dynamically allocated and freed pools of integers
protocol-constants.h	Constants related to the Guacamole protocol
protocol-types.h	Type definitions related to the Guacamole protocol
protocol.h	Provides functions and structures required for communicating using the Guacamole protocol over a guac_socket connection, such as that provided by guac_client objects
recording.h	Provides functions and structures to be use for session recording
rect-types.h
rect.h
rwlock.h
socket-constants.h	Constants related to the guac_socket object
socket-fntypes.h	Function type definitions related to the guac_socket object
socket-ssl.h	Provides an SSL/TLS implementation of guac_socket
socket-types.h	Type definitions related to the guac_socket object
socket-wsa.h	Provides an implementation of guac_socket specific to the Windows Socket API (aka WSA or "winsock")
socket.h	Defines the guac_socket object and functions for using and manipulating it
stream-types.h	Type definitions related to Guacamole protocol streams
stream.h	Provides functions and structures required for allocating and using streams
string.h	Provides convenience functions for manipulating strings
tcp.h	Provides convenience functions for establishing low-level TCP connections
timestamp-types.h	Type definitions related to Guacamole protocol timestamps
timestamp.h	Provides functions and structures for creating timestamps
unicode.h	Provides functions for manipulating Unicode strings
user-constants.h	Constants related to the Guacamole user structure, guac_user
user-fntypes.h	Function type definitions related to the guac_user object
user-types.h	Type definitions related to the guac_user object
user.h	Defines the guac_user object, which represents a physical connection within a larger, possibly shared, logical connection represented by a guac_client
wol-constants.h	Header file that provides constants and defaults related to libguac Wake-on-LAN support
wol.h	Header that provides functions and structures related to Wake-on-LAN support in libguac