While investigating messaging protocols for communication between devices and systems, I collected a list of articles, documents, presentations and specifications. Because this might be useful for other developers, I decided to share it here. The protocols I investigated are AMQP, MQTT, CoAP, and STOMP.
Update: I received a link to a series of articles about the essentials of MQTT, which I added to the list.
AMQP
MQTT
CoAP
STOMP
Comparisons