https://arstechnica.com/gadgets/2019/04/microsoft-kills-off-the-book-store-you-probably-didnt-know-it-had/

For reasons that were never entirely clear, Microsoft released a book store for Windows 10 back in 2017. The Edge browser was given support for EPUB books, and the Microsoft Store had a book category, offering a range of free and non-free books. No longer: yesterday the company ended book sales entirely.

We can't imagine the book store was ever a major business. Combined with the decision to abandon its Edge browser and switch to Chromium, it's not altogether surprising that maintaining support for electronic books was more effort than Microsoft wanted to make.