Microsoft has a vested interest in Barnes & Noble. That’s why the company invested $300 million into the book retail giant earlier this year. The deal resulted in both companies spinning off the Nook business into its own subsidiary that Microsoft can get a piece of. Now its investment is starting to pay off.

Nook Media announced today that the Nook app is now available on Windows 8 PCs and tablets. As part of its deal with Microsoft, the Nook team worked closely with the guys at Redmond to deliver an app that takes advantage of all the new features in Windows 8.

“NOOK delivers a comprehensive digital reading experience in a Windows Store app. Barnes & Noble took advantage of the new Windows user interface to combine an immersive reading experience with the ability to easily browse, search and purchase from their extensive content catalog. As consumers continue to embrace digital reading, the NOOK app is a strong addition to the Windows 8 experience,” says John Richards, senior director of Windows App Marketing for Microsoft Corp.

The Nook app promises Windows 8 users over 3 million books, 100 bestselling magazines, and a growing collection of graphic novels and comics. It also features a new “paper” UI that was “created to minimize distractions and encourage deep content interaction and enjoyment.” It also features a personalized “Daily Shelf” that organizes all the latest digital magazine issues and books into one place.

It seems pretty obvious now why Microsoft didn’t build its own reading application for Windows 8. The company’s investment in Nook insured that Barnes & Noble would support Windows 8 with its library of content.

Between Nook and Kindle, Windows 8 users will have plenty of reading content available to them. Besides, Xbox Books just doesn’t have quite the same ring to it as Nook.

Windows 8 users can download the Nook app today from the Windows Store.