I agree with you about the importance of transactions in relation to the create and update of any CRUD api. I happened to ask something similar here. (Btw, I would have used an array of futures for the user saves and flatten them, so the saves are not sequential).
But there is indeed a lot to cover for server side swift. Perhaps it might be better to have additional supplementary books to the basic one that will cover more depth in specific areas. @0xtim?