Microsoft wants you to try the Microsoft eBook Store, and to make that first purchase easier they are willing to offer you your first book free.
If you buy an eBook in the Microsoft Store for $4.99 or more you will receive $5 credit, which can go to further purchases such as apps, other books, movies or TV shots.
The Store has a wide selection of titles including multiple books from the New York Best Seller list. Books can be read in Edge on your PC, but also in Edge on iOS and Android.
eBooks are only available for US-based users. Browse the collection here.