Waaay back in 1973, Charlton Comics (defunct since 1985, though some of its superhero characters have been adopted by DC Comics) published a guide for comics creators - sort of a "how to break into the industry" book. A hefty dose of scans were posted at the blog Hairy Green Eyeball II some time ago.
Some of the book's advice is out of date, like when creators are advised not to focus on their own stories, as "ideas come from within a company, either from the staff or regular free-lance contributors." (Admittedly, this is still true at big superhero and kid's comics publishers, but these days there are plenty of companies willing to back original stories.)
Either way, it's worth a look to see how comics were produced in the days before digital art programs and sizable indie publishers!