The general rule in scanning photos for inclusion in a print book is that they be scanned at a minimum resolution of 300 dpi. What’s important to understand is that means that the quality of the scan will be acceptable if it is printed at exactly the same size as the original. A 4”x6” photo scanned at 300dpi can be printed at 4”x 6” or smaller in the book. But that’s only part of the story. If you want to enlarge the photo size in the book the original must be scanned at a much higher resolution. The Scantips.com website gives a good summary of the basics of the relationship of scanning dpi and print size in an article Pixels, Printers and Video – What’s With That?
When you want to include a large image in your book, Scantips, in an article, What About Film Scanners?, recommends working with the negative of the photo whenever possible. They advise, “The good answer for creating large images to print is to use a film scanner to scan the film instead of the prints. These are often called slide scanners, but they scan both slides or negatives. Most are for 35 mm film.” What’ important to realize when you do scan negatives is that they are often very small. For example, a 35 mm negative is 1.4 x 0.9 inch. As a consequence you will need to scan the negative at a much higher resolution so that you will get acceptable quality for the enlargement you will use in your book. Scantips recommends scanning the negative at 8 times the desired 300 dpi resolution for the printed image, or 2400 dpi.
Making sure that you get the original scans right will assure that you won’t be disappointed with the quality of the photos when it’s time to print your book.