When you press the "read" button in CC's book details, or long-press on a book when not connected to the content server, a dialog will open offering all the installed apps that say they can open the book. There is a checkbox in that dialog that tells CC to remember your selection. There is a menu line that tells CC to forget all remembered defaults.
As for which reader app, i suggest you try them. All are quite good but they do differ in ways that appeals to personnel preference. I use fbreader. Others prefer moon+, and others cool reader. There are others.