Obviously I haven't read much yet but skimming through it for 5 minutes I am impressed with what I see. Great work!
One thing I feel is missing, a PDF preview of a few pages so I can see how it looks in the digital edition. Not a huge deal so don't stress about it but I always like to preview ebooks for some reason. I may be alone here tho :)
Really though great work. I am especially impressed at the effort you put into getting a dev environment "up and running" on Windows, Linux and macOS.