Windows 10 Mobile has had a few ebook readers over the years, but here's another option. Fiction Book Reader Limited UWP also works on your Surface PC, of course, and there's a sync system to keep your reading progress fully synced up, albeit behind a subscription system.

From the Store description:

Fiction Book Reader is an application for reading e-book on Windows 10. A distinctive peculiarity of the app is pleasant design and ease of use of the rich app functionality. Our goal is to create the best application for reading book on Windows platforms.

Some of the promo screens show this in action well, albeit with a Russian ebook! Note that Fiction Book Reader Limited UWP is no fly by night application, having been around for a while (though not covered here) and having been through numerous updates.

Browsing ebooks on a phone and (right) looking at a (Russian) book's details.

There's a choice of backgrounds and fonts, as you'd expect. And no, this image isn't mirrored, that's the way cyrillic script is written!

Loads of settings to configure the reading experience to the finest detail, and all presented in a dark theme to save power; (right) there's a sync system, though it looks like that's subscription-based. This is understandable, given the need to maintain a server to handle the sync requests through the year.

You can grab Fiction Book Reader Limited UWP in the Store here.