calibre includes a built-in ebook viewer that can view all the major ebook formats. The viewer is highly customizable and has many advanced features.
You can view any of the books in your calibre library by selecting the book and pressing the View button. This will open up the book in the ebook viewer. You can also launch the viewer by itself from the Start menu in Windows or using the command ebook-viewer in Linux and OS X (you have to install the command line tools on OS X first by going to Preferences->Advanced->Miscellaneous).
You can change font sizes on the fly by using the font size buttons . You can also make the viewer full screen by pressing the Full Screen button . By clicking the Preferences button , you can change the default fonts used by the viewer to ones you like as well as the default font size when the viewer starts up.
More advanced customization can be achieved by the User Stylesheet setting. This is a stylesheet you can set that will be applied to every book. Using it you can do things like have white text on a black background, change paragraph styles, text justification, etc. For examples of custom stylesheets used by calibre‘s users, see the forums.
You can look up the meaning of words in the current book by right clicking on a word. calibre uses the publicly available dictionary server at dict.org to look up words. The definition is displayed in a small box at the bottom of the screen.