If you mean ebooks, then depending on what country that you are in you might be able to buy or download ibooks from Apple, either via iTunes on your computer or directly on the iPad in the iBooks app (you will need to download and install the app on the iPad) via the Store button at the top left of its bookshelf.
You might also have access to other ebook apps/stores e.g. Amazon's Kindle (their ebooks are in the Kindle section of their site and you can link the Kindle app on the iPad to your Amazon account), Kobo, Nook etc.
ebooks tend to have digital rights management on them which tie them to the store's own apps/ereaders (i.e. Apple's ibooks can only be read in the iBooks app, Amazon's ebooks in Kindle apps and ereaders etc), so before you buy an ebook from a particular store check whether they have an iPad app or whether they can be read in a different app (the store's site should have details).