Why not Scribus? It's FOSS, mature, and actively used for book layouts by others. It comes up in searches for "free InDesign alternative" and "foss book layout" (it's mentioned in every search result that I viewed [not many]). I mean, perhaps it didn't fit the story that showed the blog post to be used to trail the book, but I'd have thought it would be mentioned.
Do you have experience with it yourself? I haven't tried it for several years, but it was not at all usable last time I did. For a project like this, I think it's a good choice to go with something more declarative over a WYSIWYG tool. Latex would have been a very good choice, but only if you already happen to know it or have an interest in learning.
Probably about 3 years since I used it last for this type of work.
I follow one of their support groups and people mention they publish newsletters, pamphlets, and books with it.