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

[detail level 12]

include
mqtt.h	Declares all the MQTT-C functions and datastructures
mqtt_pal.h	Includes/supports the types/calls required by the MQTT-C client
src
mqtt.c	Implements the functionality of MQTT-C
mqtt_pal.c	Implements mqtt_pal_sendall and mqtt_pal_recvall and any platform-specific helpers you'd like