Photos may appear darker in the printed book than what you see on a back-lit, bright computer. This has to do with the brightness or luminance, of your display. You will see the same problem if you print photos with another print shop such as FedEx or CVS photos.
Consider adjusting the luminance of your display to 100-120 cd/m2.
If you believe that there is something wrong with the photos in your Qeepsake Book, please feel free to reach out to firstname.lastname@example.org and one of our agents would be happy to look into it for you!