To set your default book, go to the 'My Action' tab and then click the settings symbol in the top left corner.
Click 'Manage Books' at the top. It will default to "primary." Then select any book from the available books list. If you use multiple books or like to use a certain book to compare lines, switch over to the 'Compare' tab to add/edit your books.
When viewing a game's 'Preview' page, you can scroll down and see the odds for your primary book and your compare books under the 'Compare Odds' section.