If OP is used to working with HTML/CSS, it will probably save him a lot of time and energy to do basic formatting in HTML/CSS than to learn how to design a professional-looking PDF document. He's a programmer and novelist, not a designer.
In fact, he's not even aiming to design a professional-looking PDF document, just a throwaway printout for proofreaders who don't know how to parse Markdown.
Why not use something like http://github.com/susam/texme as a starting point? Really easy to turn any Markdown document into a rendered HTML with a single line of code in the header.
This rendered HTML could be converted to PDF and printed or the self-rendered HTML itself could be printed directly.
There are a whole bunch of similar tools written in all sorts of languages, and OP just chose what he's most familiar with. It just happened to be different from your favorite toolchain.
It also looks like OP's book was split into several Markdown files, one for each chapter. So he would have needed some sort of build script anyway if he wanted to use texme on the combined document. He would also have needed more than a single line of code in the header, since he wanted some custom styling for blockquotes and code snippets.
In fact, he's not even aiming to design a professional-looking PDF document, just a throwaway printout for proofreaders who don't know how to parse Markdown.