There are two phases to preparing your photos and illustrations for a book. First, the images and artifacts must be scanned properly. Then the files are prepared in a photo editing program such as Photoshop. The quality of your image output can only be as good as the input. For example, images from internet websites are generally displayed at low resolutions, usually 72 dpi, making them unfit for reproduction in a book. Professional printers require files to be submitted at 300 dpi (dots per inch).
Investigate your scanner settings and scan photos, documents and artifacts at a minimum of 300 dpi. If the image will be scaled larger than the original, scan at 600 dpi, or even higher.
Generally, scanners and cameras transfer files in the .JPEG file format. This is a “lossy” format, meaning you will lose some of the image clarity each time you manipulate and save the image. To prevent this loss in quality, after scanning immediately open the file in Photoshop and save as a .PSD (Photoshop document), which is lossless format that will preserve your original safely.
Be sure to keep these original, unmodified scans for archival purposes. When you work with an image, rename it and save it as a working copy. This way, you never need to return to the scanner.
Photoshop is a complex, comprehensive, sometimes bewildering software program, and if you are just beginning to learn its many facets, it can take years to master. Truly, if you are a beginner, it would be far more efficient to get experienced help at this stage, especially if your photos are not in optimal condition. Your book should look professional, and it will benefit from the professional touch of a skilled photo editor.