Human Made has released a white paper called Talking to 25% of the Web that makes a compelling case for the WordPress REST API along with an in-depth technical review that includes case studies.
In addition to describing what makes for a REST API and why it’s useful, the paper also digs into headless CMSs generally and how they may be utilized. The paper was written by Tom Willmot and Joe Hoyle, with consultation from Ryan McCue and Daniel Bachhuber, and additional writing and editing from Siobhan McKeown.
This is a super useful resource for learning about the REST API. I highly suggest you read it. I’ve learned a lot from it — and had many things clarified for me — despite having already researched the REST API a good bit.