FAQ for
P.S. I Love You (2007) More at IMDbPro »

Yes. The movie is based on the 2004 novel of the same name by Cecelia Ahern.

There are a few differences between the book and movie. In the book, both Holly and Gerry are Irish and the entire story is set in Ireland. As for the characters, in the book Holly had many siblings, whereas in the movie only one (her sister). The character William, who Holly meets toward the end of the movie, was not in the book.