If I've proven anything in my life, it's that one can make books available without going through a large publisher. I'll briefly share how I've done it for those who might like to do so themselves.
I started publishing print books on a whim and without any previous background in the industry or craft itself. In the course of printing my own work, San Antonio Review and others' work, I've experimented with a variety of desktop publishing, graphic design and other tools.
Requirements: Easy to Learn & Free
Given my amateur abilities, I've tried to find easy-to-use solutions that have minimal cost.
I currently rely on the following software solutions for different aspects of the publishing process.
Adobe's products, of course, are still the leader here for quality, flexibility and robustness, but resource usage, financial cost and the learning curve are high. GIMP is free, but still difficult. Canva is easier-to-use but certainly lacks in flexibility and consistent quality, in my experience. Still, I use Canva for most of my work.
I've found Pressbooks to be the easiest to use to layout print matter. Affinity Publisher is another product I'm fairly certain could be highly useful, but I haven't learned enough of it yet to fully utilize its capabilities. Both are far cheaper than Adobe-branded alternatives.