[throwaway729]

I don't agree.

Editing for content or style?

Style editing is usually detrimental IME. Especially when the style editor doesn't understand the content and is just applying typographical rules.

And content editing is usually done by fellow professors or students. Which if the book is open source will happen anyways as new professors use the book in their courses and notice errors.

[cycomachead]

Publishers provide style, content/grammatical editing, domain experience and pedagogical review.

Most professors don't have strong backgrounds in pedagogy and even those that do benefit from having others edit their work.