Fiction by Harlan Coben.
At the beginning of this book, a mom picks up a roll of film developed at the local Photo-Mat and discovers a picture inside that should not be there. (Obviously this was written over ten years ago. Just go with it. Google the phrase "film developing" if you need help.)
Since I'm actually old enough to remember the pre-digital age, I've experienced the fun surprise of picking up photos. What will be in the packet? What if you found a picture there that changed everything?
This was a great thriller. I really had no idea what was going to happen at the end!
I also read recently by this author: Six Years