Definitely the books that were given to KatieAnn by guests. I asked my guests to write a letter to KatieAnn on the inside cover explaining why they chose the book for her. When they come to visit KatieAnn in the future, they will have "their book" to read to her!
I tried my best not to over-do the "baby duck theme". By creating a party that didn't obsess over everything baby, it saved me money because I could use things I already owned or found relating my theme to create the perfect gender reveal!