I read “Effective Go”[0], but what really helps me is browsing through the sources of various Go projects and libs in Github. I also keep a tab of Go’s packages docs open [1]. And of course there’s StackOverflow. In general I prefer learning from source code rather than books.
Although the question is not directed at me, I've found exercise websites to be extremely valuable. I value learning/gathering experience by reading code, but I find the hands-on approach more stimulating.
My favourite resource right now is exercism.io. Although some tracks have too much of a high students/mentors ratio to be effective/practical in mentored mode (or to activate the mode at all), going through all the exercises (and community solution) is still a very effective way to learn a language (along with a textbook, of course).
Codewars is another interesting idea, however, it's much more grounded on computer science, which is not very well suited to just learning a language.
0 - https://golang.org/doc/effective_go.html 1 - https://golang.org/pkg/