Best tools for writing ebooks in 2020?

I've been having a ball the last few years writing all kinds of ebooks with LaTeX - mathematics, programming, documentation, literary, illustrated notebook, diary etc. In my case, TeXShop from the standard Mac TeX distribution. It took a few months to become familiar with the language and the common packages, now it's all easy. The TeX Stack Exchange is awesome.

I wrote mine (https://gumroad.com/l/dDXav) using markdown.

I used pandoc to create the epub from markdown, a free online converter to convert epub to pdf, and kindlegen to convert epub to mobi.

It was really easy. I didn’t pay anything to make the book.

There are a number of Pandoc ePub templates floating around GitHub.

Looking forward to the answer to this question!