Here’s a great video which describes the process for creating an accessible textbook “quickly” via scanning. The best tag line is – “The problem is not that Svetlana is blind. The problem is that this textbook is only published for students who see.”
<p Ironically even almost all textbooks actually begin digitally as a Word or InDesign/Quirk file, this scanning still occurs because it takes publishers 2-3 weeks to send the original PDFs and that's too long for students to wait.
Did I mention that it is captioned and includes audio description for the visually impaired?