In addition to the aforementioned apps, I use WordBook XL, Wikipanion, iBooks, and Free Books.
WordBook XL is a handy tool and provides as a dictionary and a thesaurus. I had to pay for this app after the iPad 1 launch , FYI.
Wikipanion is great for on the go information and basic background information of numerous subjects. Be careful using it as a reference, as some professors frown upon such action.
Check iBooks and Free Books, as they offer some classics for free. Instead of paying $6.95 for a book you might read once, check the lists to see if its free. I was surprised to see books I had purchased a year or so ago, free.
Also, there are apps tailored for specific disciplines such as: science;the Periodic Table app, history and law;Constitution app and a free advanced calculator app; math and science.
I like the iPad and think it is useful for many things, but I bought a wireless keyboard, because I cannot type quickly on the touch screen.