Some info over in the book house
Basically nook/Kindle/Kobo are all good. Big part of it comes down to whether you want physical page turning buttons or not. nook has buttons, others are all touch screen. Kindle is safest if you are worried about B&N going under, but I'm pretty sure they are OK now...I think.
Not sure about audiobooks, pretty sure they all have that option. My old ass nook plays mp3s so I can't imagine newer ones don't, but I have zero experience with any of them in that regard.