I will mention that college level English departments should not need to teach basic English grammar. I would expect that anyone requiring help in that area would be directed towards remedial courses (or a different discipline).

My (small) experience with the English department , was that it did indeed teach English literature... (although really you should be getting an education in not just English literature, but literature in general). Of the few classes that I took, I found it to be very multidisciplinary. It bridges philosophy, sociology, history and art (at the very least).

In reality, very few people who graduate from college actually end up publishing a book, let alone a one that is really worth reading.

I think it will be very hard for you to pull together specific data about what major will write a well evidenced book in some subject, but it seems to be begging the question to say that a computer science major is more likely to write a book on public policy than English or Business majors. It is kind of a speculative claim.

I personally consider college education to be a process of learning how to learn.

Yes you are forced to focus in a specific discipline, but the end result of an English undergraduate degree is not to be an expert able to discuss the western literary cannon (that is the point of a PHD). I would expect an English undergraduate to spend his or her time reading books mindfully and learning how to recognize the themes, ideas and relevant social commentary from the book.

That sort of meta-cognition is what makes you good at learning in general. Being good at generalized learning is much more useful that having abstruse knowledge of Chaucer...