I've used flask, rails, and padrino (sinatra).
Flask was my favorite but is hard to keep organized without much of a toolkit or recommended dev patterns baked in. I recognize that blueprints really help, but a consistent cross-project organization would be nice.
The transition to padrino was welcomed due to a lot of basic structure that is consistent in all projects, but unsustainable due to a functionally dead community (#sinatra has 25 people at max and nobody talking, #padrino has maybe 8).
Rails is far too much bloat to just be serving JSON (benchmarks show that sinatra is 4-8x faster...) and implies TOO strict a structure to development (also, being a python person...I don't appreciate magic).
What do you recommend?
As for a guide... There's a guide for LoopBack specifically here: http://docs.strongloop.com/display/LB/Getting+started+with+L... That framework was written specifically for writing REST APIs.