Neither the Kindle or Amazon mobile apps allow you to buy ebooks. This is the reason why, and what you can do to get around ...

You can buy books directly on your Kindle by accessing the Kindle Store and tapping the Buy button. You can also buy books from Amazon's website by searching for the book, selecting the Kindle option, ...

while simultaneously expanding its Kindle e-book store. The result, at least for Kindle users, is easy and reliable access to an Amazon storefront packed with ebooks from all your favorite authors.