If you're using a device that can use either the Google Books app or the Kindle app from Amazon, there's no reason you can't use both apps on the same device. Depending on the hardware that you're using, you'll find some distinct differences between the two applications.
Types of Apps
Both Google and Kindle offer several different apps for reading and syncing your books. You can get Kindle apps for an iOS, Android, Windows or BlackBerry mobile device, a desktop computer running either Windows or OS X or even your Web browser. Google Books is available on fewer platforms -- you can use Google Books on an iOS or Android mobile device, or online with the Web Reader.
Digital Rights Management
Both the Google and Amazon e-book stores use digital rights management technology to prevent book piracy, but they employ it differently. Amazon uses a proprietary DRM with a proprietary file type, so your files only work in the Kindle app or on Kindle devices. Google Books uses Adobe DRM with ePub files. This model allows you to read your books within the Google Books app and also transfer files to any e-reader compatible with Adobe DRM.
Selection & Pricing
Google Books claims to have over 5 million e-books available as of April 2013. This number includes both new books and free books within the public domain, such as Jane Austen's "Pride & Prejudice." These same public domain books are also available in the Kindle Store, but overall the store offers fewer titles -- just over a million. Additionally, Amazon Prime users can rent books from the Kindle Lending Library; there isn't an equivalent service for Google Books.
In terms of interface, the Google Books and Kindle apps use a similar aesthetic. Both allow you to change between three color schemes: black text on a white background, white text on a black background and sepia. Both allow you to change the text size and line height. You can browse your book collection in both apps. With Kindle, you can buy new books and read them without leaving the app, while the Google Books app takes you to the Google Play store to make purchases. In the browser reader apps, you can switch between a two-page view and a single-column view, with the same text and color changing features as the mobile apps.
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