This is one other thing you may want to consider having. There are a few different options:
create and print the book using your engagement pictures and other pictures you have as a couple. There will be a lot of clear white pages for your guests to write.
You could use a clear white book, bring an old Polaroid camera with instant prints for guests to use. They will be taking pictures of themselves, sticking it in the book and adding their words.
There is also an option of bringing a photo booth with instant printing. It may also be an additional entertainment for the wedding party. The pictures from the photo booth could be added to the book.