If you are backing the "Book 1 + Book 2" tier and you want to have access to some of the rewards, simply add the value of the reward to your pledge. There will be a space in the survey to note down that you have done so and to say which reward you wanted. The value of the additional items are as follows:
Stickers - $5
Bookplate - $10
Prints - $35
All physical copies of Book Two come with the bookmark included.