why can i bye a book from itunes libery but it will not open on this pc

The iBooks sold at the iTunes Store are only for iOS devices such as iPads and iPhones. They cannot even be read on a Mac, much less a PC!

If you wish to download books to read on your PC, get the Kindle Reading App for PC, and buy your e-books from the Kindle Store.

The Kindle books can be used on PC, Mac, Kindle readers, and iOS devices, by use of the corresponding apps.