Depends on what features you want in your book. Out of most of the free/open source tooling, pandoc is probably the best. I've written multiple books with it a

Depends on what features you want in your book. Out of most of the free/open source tooling, pandoc is probably the best. I've written multiple books with it and have also used proprietary tooling of publishers. I have a few plugins I use to customize my books, but have yet to find a tool that I wouldn't need to customize.