To set your default book, go to the 'My Action' tab and then click 'My Books'. From there, you'll be able to add, edit, and remove sportsbooks to your liking.
If there are any books under 'My Books' that you do not want, just click the edit button on the right-hand side and then click the red minus symbol. To make one of your books your 'Primary Book', click and hold the three lines and drag the book up to the top.
By doing this, you'll always be able to see the best odds available to you when tracking your bets. The 'Best Odds' view will show you which book is offering the best odds for each wager option from the books you've chosen. You can also toggle to the left or right to view the odds from one of your particular books.