well given all the politicians release books bragging about themselves and they do make sales I assume there are some buyers who want to do more than show them off

(also ik you're joking but a lot of otherwise very smart people do end up right wing for inane reasons)

What word would 'left-book' be replacing?

Double-plus-good isn't used because it's a new word. It's used because they can't use "brilliant", "excellent", "fantastic", etc...