You only need someone to buy your book, you don't need them to read it ;)

You could extend that argument about aesthetic design to any application though, but I think that may be framing it in the wrong way. It's (usually) not much extra effort to, say, choose a nice font or use a pleasing colour scheme. I don't think good aesthetic design should be reserved for the masses or recreational media. Good aesthetic design isn't "beneath" anything; I don't see a reason to justify bad aesthetic design in educational or instructional resources.

> "Good aesthetic design isn't "beneath" anything"

I didn't say it was, however if I'm picking up a technical book I'm not fussed what the cover looks like, because I'm already motivated to learn the information contained in the book. It's best if that motivation to learn exists before someone picks up a technical book. If it doesn't the chances of getting through the book are slim to none, regardless of what the cover looks like.