@mbostock offers a five minute introduction: link 
Observable notebooks can now be downloaded and embeded: post
Sentiment analysis with tensorflow observable
OpenGL Shaders drawing polygons observable
Introduction to Generators observable
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This looks like the next logical improvement on the bl.ocks.org d3js.
I'm sad that "observable" is so quickly overloaded with meaning. Here we have Mike Bostock's latest project. Elsewhere I've been reading bits and pieces about observability as an essential ingredient to keeping distributed systems running.