If you have a Mac and want to write and publish a book, give Scrivener a try. Scrivener is a writing app that allows you to outline, research, write, and publish your book in a single app.
Scrivener 3 was recently released for Mac, and it allows you to publish PDF, EPUB, and MOBI (Kindle) books by choosing File > Compile in Scrivener. I tried compiling an EPUB book in Scrivener 3, and it looked good without me having to do anything. After using Scrivener 3 I lost enthusiasm for working on Tome Builder because Scrivener 3 does everything I wanted Tome Builder to do.
Scrivener also has iOS and Windows versions, but I haven’t used them. The Mac and Windows versions have 30 day trials, which you can download from the Scrivener site.