The standard ebook format is EPUB, and there are several applications you can use to view them on a Mac, Windows or Linux system.
If you're using a Mac running version OS X 10.10 (Yosemite) your best option is to use the iBooks application. More information on using iBooks for Mac or iOS is available.
Mac, Windows or Linux
If you're using a Mac running an earlier version of the operating system, Windows, or Linux you can use the Readium app for Google Chrome. This is an extension to the Google Chrome web browser which allows you to read standard EPUB files. You can also copy and paste text, which is useful for keeping track of corrections.
Readium requires the Google Chrome browser, which is available for Mac, Windows and Linux.
Alternatively, you can use Adobe Digital Editions to view EPUB files. It isn't as good at displaying ebooks as iBooks or Readium, particularly complex tables, but is okay for text proofing.
Adobe Digital Editions requires Mac OS X 10.8 or later, or Windows 7 or later.