Slate has polled various authors on what fonts they compose in. Of course, the majority winner was Courier, ye olde typewriter font that is so squat and square on the page.
I can not stand Courier; it pains me to even look at it. If someone emails me a manuscript in Courier the first thing I do is change the font. Otherwise, I just can't read it.
Traditionally, submission guidelines have requested that manuscripts be presented in Courier 12. Courier is a monospace font, which means that each letter is exactly the same size. Editors, supposedly, like monospace fonts because it makes it easier for them calculate the length of a book. Of course, few books are actually printed in Courier, because it's not very aesthetically pleasing.
I write in Times New Roman 11, which I then bump up to 12 before printing and sending. So far no editor has complained. And every time I see something in Courier, I shudder. For some reason, the way in which those blocky letters march across the page makes the words almost impenetrable to me.
But then I'm fussy about fonts. I have not bought books because of the font, or because I didn't like the line-spacing. I've even put books back on the bookstore shelf because I didn't like the paper the book was printed on.