Microsoft is discontinuing the books section in the Microsoft Store, according to a support page updated today. The demise of the books tab is effective immediately, meaning you can no longer buy, rent, or pre-order books through Microsoft's platform.
The ability to buy books from the Microsoft Store, which you could then read using Microsoft Edge, was introduced with the Windows 10 Creators Update, about two years ago, so it's been a short-lived platform. If you have any books you've already bought or rented, you can read them until the end of the rental period, or until July 2019, when they will be removed entirely. On the bright side, you'll get a full refund of the original price of the purchase when they're no longer available. If you have pre-ordered any books releasing after today, your pre-order will be canceled and you won't be charged.
Microsoft's e-books were available through its Edge browser, which has some incoming major changes with the shift to Chromium engine. This, combined with the fact that the platform likely didn't gain a lot of traction, given the existence of well-established players such as Amazon, might have contributed to the death of the service.
Microsoft has given up on a lot of its services in recent months and years, including Groove Music, which has even lost the ability to stream songs from OneDrive recently, so this move shouldn't come as a huge surprise. If you'd like to keep reading your books digitally, you'll have to look elsewhere.