If you deal at all with EPUB (electronic publication documents), EPUB Viewer for Mac OS X (10.6 or higher) from Enolsoft is a tool worth checking out — IF you need lots of bells and whistles. A demo is available for download, and you can buy the utility for $2.99 at the Mac App Store.
What's EPUBL? It's a free and open e-book standard by the International Digital Publishing Forum (IDPF). Files have the extension .epub. EPUB Viewer for Mac is an EPUB eBook reader that lets you open and read non-DRM protected EPUB formats on your Mac.
It has zoom-in, zoom-out, bookmark, scrolling, print and other features. You can also use it as a Mac EPUB to PDF Converter to convert an EPUB document to a PDF document. Converting EPUB docs to the PDF format conveniently retains the original elements, including layout, text, images, hyperlinks, tables, fonts, and graphics.
What's more, you can open and view multiple EPUB files in tab mode, as well as play embedded media files, audio and video in EPUB files.
All this is very handy. However, if all you want to do is read EPUB docs, there are some free alternatives you should check out, including Stanza and Calibre. If you need more features, check out Bookle, a $9.99 tool available at the Mac App Store.