I don't mean this in a derogatory way, but you are insane.
I've written a couple for MK and SV and edited a few more conference books and a few more compilations of submitted chapters in an attempt to create a cogently thematic book, and I hated the world, everyone in it and myself. Never again.
...or am I wrong?
We use Markdown for authoring (one of two dialects: either Leanpub Flavoured Markdown or Markua).
We automatically produce PDF, EPUB, MOBI (what you were saying, essentially), but we also produce InDesign export. If you have a designer, give him/her the InDesign files. You can then produce exactly what you want as a finished product for print, with as much design as you want. But while the book is in-progress, nothing beats being able to click a button to publish.
...and if you don't want to do the InDesign work, you can just click a button and get a print-ready PDF file automatically produced by Leanpub.
I got the impression it was 100% opaque from web ui -> paying for printed books.
Do you offer paper size/weight options? And what are the prices if I already have the book proofed and am just looking for a press?
With the print-ready PDF, you can use it on unchanged places like Lulu or CreateSpace. (With the print-ready PDF, we alternate margins (wider in middle) and page numbers (put them on the outside), start chapters on the right, etc. We also omit the cover, so that you can use their cover wizard things.)
With InDesign, your designer will need to do some work. We produce ICML.
Leanpub is primarily an ebook production workflow + storefront. The print-book-ready file export is something we just give away, since our authors want it. We currently don't want to be in the business of producing physical books ourselves.
but my opinion is quite different.
i've been active in the e-book sphere for a long time, and i've studied e-book authoring quite extensively, even programming some systems myself, and i think leanpub is quite good. i liked it better when you didn't have to make a lump-sum payment in advance, but even now, i think it's one of the best systems around, especially for people who don't want to code their own solution. and leanpub gives you access to a customer-base that's more than large enough to warrant the small percentage they skim off your sales.
[0] https://michaelochurch.wordpress.com/2017/05/06/crossing-the...
Edit: here's the explanation on why you can't find a lot of his earlier material online: https://www.reddit.com/r/programming/comments/44bmmq/why_agi...
The first 11 books I wrote were published by mainstream publishers (McGraw-Hill, J. Riley, Springer-Verlag, Morgan Kaufman). While I am grateful for the advance monies paid and great support in writing, I eventually decided that I wanted the freedom to write on whatever topics I wanted and all my new stuff is self published, first through lulu.com and now through leanpub.com.
I encourage people to write a book (or books). It is a really fun process. I would suggest using a publisher for a book and then also try self publishing using leanpub.com or some other platform. Decide for yourself with you prefer.