As far as books, the "Big Kahuna" is Amazon. Of course you have heard of the Kindle reading device, but there is also a Kindle app for the iPad which allows you access to exactly the same stuff as you would have on the stand-alone Kindle reading device. The two major "brick & mortar" books stores also have reading apps. The Barnes & Noble app is called Nook (just like their reading device) and Borders has a reader that is exactly the same as the Kobo reading app.
Another major player is called Stanza. Not only is it probably one of the best apps for reading public domain material (public domain = FREE!), you can also download books from a couple of online booksellers including SmashWords which is a great site for self-published authors. In addition to that, you can also download books from Project Gutenberg which has over 33,000 public domain titles you can download for free! Most of these are the "classics" you had to read in high school, but there are also some obscure titles over there like "All About Coffee" which is several hundred pages published in 1922 and will tell you more about coffee than you ever wanted to know! There are also several other sources of free material including Random House and Harlequin (if you are into that mushy romance stuff!)
Stanza also allows you to put your stuff on one of those "clouds" like DropBox or Box.net.