I’m working on an intermediate level Elm book called Practical Elm for a Busy Developer. It covers things like server requests, JS interop, debugging & testing tools, JSON decoders, various options for creating UI & styling it (including my personal favourite style-elements), as well as many other things like the ins and outs of destructuring & pattern matching, opaque types etc.
I chose not to include the basics (Elm syntax, the Elm architecture etc.) in the book because I think they are adequately covered by other sources.
It’s currently available as an “early access” book, with some of the content yet to be included: https://korban.net/elm/book