MQTT.org
Unknown
Official MQTT protocol site with free tutorials and specs, top rec in multiple r/IoT threads
More resources on IoT Protocols
CoAP Technology
Official CoAP resources and comparisons, practitioner favorite from Reddit
HiveMQ Blog - MQTT Tutorials
Free in-depth guides on MQTT and other IoT protocols, frequently upvoted
mqtt.org
MQTT.org is the official home of the MQTT protocol, providing the specifications, version history (3.1.1 and 5.0), and authoritative guidance for developers. It also links to client libraries, brokers, tutorials, and other resources to learn, implement, and deploy IoT messaging with MQTT.
iotworldtoday.com
IoT World Today is a news and analysis site covering Internet of Things technologies and protocols, with articles on MQTT, CoAP, edge computing, security, and real‑world deployments. It offers industry insights, tutorials, case studies, and practical guides for building and scaling IoT solutions.
MQTT Essentials: A Lightweight IoT Protocol
MQTT Essentials: A Lightweight IoT Protocol - A book resource
postscapes.com
Postscapes is an IoT-focused resource hub with news, tutorials, and buyer guides on IoT technologies and protocols. It provides protocol overviews (MQTT, CoAP, etc.), architecture guidance, and practical guides on sensors, devices, and smart‑home use cases.