Disappointed that dependency management seems to be based on old vendoring technologies and not Go Modules... (caveat, I only looked at the toc and some previews... but still)
However, for the book source code, I opted to use dep to make sure it can be used by folks that use older Go compiler versions that lack support for Go modules. That being said, it should be trivial to convert the repo to use Go modules.
Why? (I don't mean that as snark; with the ease of installing and upgrading the go compiler compared to the olden days of acquiring a c compiler on a floppy disk - why not target the latest version? I[s] cgo trailing behind?)