Thanks! I usually write on niche topics, but as long as I have some enthusiastic readers I will keep writing.
I am very biased in favor of Lisp languages, but I have been getting into Swift because: easier iOS and macOS development, exciting possibilities with TensorFlow in Swift, and except for not being a lazy language, it reminds me a little of Haskell development (another language I like).
I have material for updating my Hy and Common Lisp books and when I finish updating these two books (always free updates for my books on leanpub) in the next month or two, I might write a Swift book based on my experience developing my product (which is a combination of a tool for training the effective use of SPARQL with Knowledge Graphs, and a tool for browsing Knowledge Graphs). I will float this idea to people who follow me on Twitter (@mark_l_watson) to see if there might be interest.