If the objective was not spend on design software, why not use latex?
You can tell it was built from the ground up for books, and not scholarly publications. In that respect it’s actually closer to plain TeX.
It doesn’t try to be overly semantic because books are complex beasts, and there really is no useful-but-wrong simple model that doesn’t handicap you.
Like, not all books have chapters. Some have hundred of very short chapters, with multiple on a single page.
It’s also much more “batteries included”.
Basically; trying to do a book in LaTeX you end up fighting the defaults, and for some there are NO good reliable overrides.
Take a look at the manual (http://www.pragma-ade.nl/general/manuals/ma-cb-en.pdf) page 83 (numbered) 87 in the pdf. Imagine trying To the that in LaTeX.
I agree that ConTeXt would be a better fit than LaTeX.
Yeah, agree, but a work of fiction doesn't usually have tables, figures, references, equations, etc which can be painful in LaTeX.
It does have kerning, ligatures, orphan/widow management, runs/ladder management, page numbering, chapter numbering, margins (larger closer to the spine), rules, struts, etc that all come for free without the author having to know any of that stuff.
IOW, for a work of fiction, the learning curve for LaTeX is likely limited to setting the font, setting the size, setting the output type (book, article, etc) and using bold, emphasis, underline and possible verbatim.
The learning curve for LaTeX is a cliff if you're writing a thesis, it's simply markup if you're not.
Anyway, I am happier creating native (desktop) applications than webapps, except that I don't think that there is a market anymore for them. How well is your native desktop app doing? Is there a large enough market for applications that need to be installed locally only? How do you market these?
Book layout is a very complicated topic. If you look at that image and step away a bit it leaves a chaotic visual impression. Reason behind that is that if you want a visually pleasing look, all the paragraphs have to have a similar "density". That is defined by how long the words are, the spacing in the words (kerning, ligatures, font), and the spacing between the words. Latter is all across the board, even changes within the same paragraph! LaTeX has all that typesetting knowledge included, and it will actively hyphenate words to accomplish its goal. I see exactly one word being hyphenated there.
Another thing that stuck out immediately was the font selection for the all uppercase paragraph. Some fonts are just not made to be used that way and this is one of them. These curvy uppercase letters are nice to start words, but used together it is ... ugh.
Last is that leftover sentence at the top of the right side. It "validates" the separator image, so this is somewhat of a pro/contra mixed bag situation.
Professionals could probably say even more about all this, I just used LaTeX for a couple years more often and these are things I learned (to see) on the way.