I wrote Open-Publisher: https://github.com/chrisanthropic/Open-Publisher with a focus on easy fiction book creation.

Markdown manuscript input with high quality epub, mobi, and print-ready PDF output.

It's a wrapper (your choice of Rake, Bash, or Docker) combining Jekyll + Pandoc with custom PDF LaTeX templates for print-ready (valid PDF-X1A 5x8 and 6x9) and professional epub/mobi ebooks.