Official resources
Best practices for writing idiomatic Go code. Idiomatic Go is, almost by definition, whatever follows Effective Go and the other articles on best practices by the Go team.
Go at Google: Language Design in the Service of Software Engineering
A modified version of the keynote talk given by Rob Pike at the SPLASH 2012 conference in Tucson, Arizona, on October 25, 2012.
Google style guides
An absolute gold mine of a resource. So many pages there are top-notch.
"The Go Programming Language" book by Alan A.A. Donovan and Brian W. Kernighan
One of the best books on any programming language. The examples and exercises are very interesting due to focusing on real-life applications (e.g. creating GIFs, 3D wireframes, HTTP clients, etc.).
"Go Proverbs" talk by Rob Pike at Gopherfest 2015
Similar to the "Zen of Python" for Python. It highlights the opinionated nature of the language and the suggested mindset for programming in it.
See also each proverb with a link to a timestamp in the video here.
"I Want off Mr. Golang's Wild Ride" and "Lies we tell ourselves to keep using Golang" articles by fasterthanlime
An interesting pessimistic account on Go. As said in the articles, Go has a very good marketing team, which can lead you to think it's the perfect modern language. It's good to be aware of the tradeoffs of the language and see if they're acceptable to you.
"100 Go Mistakes and How to Avoid Them" book by Teiva Harsanyi
The title of the first chapter says it all: "Go: Simple to learn but hard to master." Go has many pitfalls so it's important to know how to avoid them.
I haven't checked these out yet, but they look interesting:
"Go Class" playlist by Matthew Holiday
I haven't watched this yet, but saw it mentioned in the Go Nuts Mailing List. Matthew is currently a Staff Software Engineer for The New York Times, so this looks interesting.