When I started off with Ruby I tried reading through some freely available books (including Why's Poignant Guide) but going through the koans was by far the fastest and most effective way to learn Ruby syntax.
I found reading The Well-Grounded Rubyist and completing the koans to be a good combination. You get an understanding of the underlying concepts of the language as well as experience with the syntax and core classes.
This is an enjoyable and effective way for getting used to Ruby syntax and some basic
TDD concepts. The important point to keep in mind, however, is to try to reflect
on each modification that you make, so that you can also learn the language
semantics, and make the practice more than just "filling in the blanks".
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https://rubymonk.com/ is another good one. The theme got a little tiresome after a while but the lessons really forced me to mess around with Ruby in the console. I learned a lot.
There are also Go koans: https://github.com/cdarwin/go-koans