The Nook app was recently updated to read EPUB files:
"Import ePub and PDF files from your PC or SkyDrive to access your favorite content in one place."
Also, take a look at the freda app:
"Freda, the free ebook reader. The popular ebook reader for the Windows Phone platform (and Windows Mobile before that) has arrived on Windows 8. Freda can read EPUB (DRM-free), FB2, HTML and TXT format books. The app understands EPUB formatting information (bold/italic/large text, margins and alignment) and can display images and diagrams in books."