Sure there is the Kindle Fire and Nexus 7 but the iPad 7.85(iPad Mini) I think will outsell the regular iPad and possibly the iPhone. Regardless of speculated specs, the iPad Mini will do so well because Apple has deals with Major Book Companies like McGraw Hill, etc. having a MAJOR foothold into education. Which means this iPad will be in practically a majority of schools in the US with in a 2 or 3 year span.
With a $250 iPad, its just cheap enough to afford, thin and small being durable(Gorilla glass), be portable but yet capable for normal reading(book replacement) and if apple reduces the thickness of the screen I wouldn't be surprised if they increased the battery life of the iPad mini to something like 15 hours(compromise slight weight gain for life) which makes it very feasible to replace a book.
Long battery life, affordable, thinness, durability, major book companies support, apple reliability. = Apple has a Winner
I'm coming from the fact that apple has major book company support and apple reliability. Those two together is the big bang and will change the tablet world into something else, star trek ish as in everyone owns one.. that is the standard.
Edit- With that said if Apple Add's NFC to the iPad Mini it'll also take off.