1. Scan it in .tif, if possible
2. Scan at the dpi (dots per inch) at the size you want the photo to be printed, at 300 ppi. For instance, if you have a 4×6” photo, and want it to appear in your book at 4×6 inches, then scan it at 300 ppi at its original size. (This will give you an image of 1200 x 1800 pixels, or 2.16 megapixels.)
If you think you might want to print your photo larger than 4×6, then scan it at 600 ppi. This will give you 2400 x 3600 pixels, or enough pixels to print a good quality picture at 8×12 (a full page for a book). You can always print a picture smaller than your original scan, but you can’t go larger without losing quality and sharpness, so make sure you get all the pixels you need when scanning.
If you will be cropping the photo significantly or zooming in to a small portion of your scan, you will also need extra pixels, so scan at a higher resolution.
I got a lot of this info from: picturesandstories.com/news/2013/4/24/how-to-scan-photos-for-print