What are your favorite FHIR learning resources?

Hi, everyone, 

We in InterSystems Learning Services are compiling and reviewing different non-InterSystems materials for learning various technologies. To this end, can I ask:

-What are your favorite learning resources (books, courses, etc) that have helped you with the development and implementation of the FHIR standard and why do you like them? 

Thanks, Jamie Kantor

Certification Manager, InterSystems