You'll want map_err if you have function A that returns a Result with one type of error, and you need to call it in function B which is required to return a Result with a different type of error. I did serious Rust programming for like 2.5 years before I recall having to use map_err. It'll be there if you ever need it, but it's not necessarily something you have to learn right away to get productive.

While you're changing tactics, though, I'd suggest thinking about starting a real project in Rust rather than working through exercises. I never actually worked all the way through the official Rust book myself. I didn't seriously make an effort to learn Rust until I had a real project I wanted to do in it.