I want to reprint an iPhoto book from 2010

Apple keeps the pdf files for only 30 days. To reorder you'll need to do so from the library that was used to create and order the boot originally.

For future books you might consider doing what I do: create a new, separate library for each book. One can use iPhoto Library Manager to move the photos between your working library and the book library. If you use Photos PowerPhotos will do the same job.