The Go standard library is very well written and depending on which parts you read you can learn about lots of things like file operations, HTTP, crypto, etc.
It's easy to read it all on the web, the docs are here: https://golang.org/pkg/ and clicking on a function name shows the source.
It's easy to read it all on the web, the docs are here: https://golang.org/pkg/ and clicking on a function name shows the source.