"The gateway drug" IMO is the Phoenix framework. It models cleanly to much of what you already know as a Ruby dev. The creator, Chris McCord wrote a book [0] for it that is one of the most well written programming books that I've come across. Chris does a fantastic job of making the information accessible and understandable. You learn by building an app of substance and apply all the concepts that make phoenix a wonderful web framework to use. Towards the end, Chris starts easing you into some of the more "hardcore" concepts and gives you the opportunity to use them on the project you are building throughout the book. That level of quality just seems to permeate the ecosystem as a whole. Others on here have made the point that the waters might start getting muddied as it gets more popular but I've had nothing but great experiences so far and would love to see it really take off.

[0] https://www.amazon.com/Programming-Phoenix-1-4-Productive-Re...