The basic problem with grinding in an MMO, after all, is that it doesn't require your full attention, especially if your goal is straightforward and you've done it before in one way or another. It's relatively easy to just put yourself on autopilot and do the job while your mind does something else. Listening to an audiobook is the perfect companion to this, because it fills up your mind, and leaves your hands and eyeballs itching to do something of their own.
Also, I'm a person that has trouble reading with my eyes. I can do it for short periods without any trouble, but with long books, I tend to fall asleep or get distracted very easily. Through Audible, I might have read more books with my ears than I have read with my eyes by now, and although I know some people must have the paper copy of a book in their hands, there's probably a large number of WoW players out there who find themselves not reading as much as they would like, and would love to know that there's another way to get their literary fix.
Just last night, I was sitting down to do my daily quests for the Netherwing faction, and I remembered I had just finished my latest audio book (Neverwhere, by Neil Gaiman, read by the author!) earlier that day while sitting in a taxi. So I stopped the game, and went to go download a new book (Path of Daggers, by Robert Jordan) and write up this post at the same time. While playing, of course, I often pause the book to read quest text, get myself out of a tricky mess with a pack of monsters, run an instance, or chat with my friends. But whenever I feel the least bit dissatisfied with the game's tickling of my attention span, I turn my iPod back on and enjoy the story. Grinding, then, lets me enjoy the audiobook even more by giving me something easy and fun to do while I listen.
The first thing you might notice when you visit audible.com is that the books seem expensive, but in fact, they're only expensive if you buy them one by one. They have different plans with which you buy "audible credits," either once a month, or all in one big yearly rush, and depending on which plan you choose, the price per audiobook credit drops to between $10 and $15 a book -- potentially cheaper than many paperbacks! Personally, I go through so many audiobooks that I just renew my yearly plan whenever its credits are used up, but of course each person should only sign up for as much as they think they'll listen to. If you love both reading and WoW, you may enjoy this as much as I do.
Edit: A small disclaimer: I have absolutely no business relationship with audible.com, and this is by no means an "advertisement" for them. Other people have mentioned that they enjoy listening to podcasts, the news, library audiobooks, and many other things. While audible.com the source of audio literature that I like personally, you may, of course, try a number of different things and find what works best, whether it's audible or NPR or something else.