There's also literally millions of free ebooks available that are part of the public domain with expired copyrights. They're mostly classics from famous authors and if that's what your looking for you will have enough books to keep you busy for the rest of your life. If you're a student this will be extremely valuable for you because you can conduct research from many different sources without having to lug tons of books around with you. The best place to start with would be project Gutenberg because they have the biggest selection available. You can search by title or by author and you have a choice of what type of format you wish to read them in. For the kindle you simply choose the kindle format and if you choose to read on your P.C. you can select html and read it in your web browser. For the nook and most other reading devices that support the epub format you have to choose the epub format. If your having trouble with this particular format you can use a software to change it over to pdf format which is supported by most devices.
There are dozens of other websites that offer free ebooks that are classics the majority of them are in the pdf format so they should work well with most devices. If you are using any of the Apple devices such as the iPad, iPhone, or iPod touch you have by far the largest selection. You have access to the free ebooks from the ibookstore and you have access to the free ebooks from the kindle store through the free kindle app for the iPad and iPhone. The same goes for the free ebooks from Barnes & Noble through their free apps for the nook as well as the free app for borders gives you access to their free ebooks for the kobo ereader.
Well in short whichever device you own there's plenty of free ebooks available to keep you busy for a long time. So just sit back kick off your shoes and start reading.