If you are running Windows 10 Creators Update, you might have noticed the new Books category in Windows Store. The Books category is a new digital category joining games, apps, movies/TV, and music in Windows Store (US only). It allows you to discover and read e-books with a simplified interface, customizable learning tools and offline support.
Microsoft today announced a limited time offer to promote the usage of eBooks on Windows 10 devices. When you buy any book in the new Books category of Windows Store, you can get $5 credit towards the next book you pay for. This offer ends on June 30.
You can find the offer details here.