As you continue to fine tune the images in your project, one of the most useful tools is Adjust.
To begin, click on your image, then the Filter/Adjust Photo button on the left. From there, click on Adjust.
When you select the Adjust option, you'll see multiple tools to make changes to your image.
The first which is Brightness. This is one of the most important tools, to ensure that the images in your book are clear when printed. Note that a computer monitor is backlit, while printed projects are not, so images will appear a little darker in print than you see on the screen. This is especially true when the image contains shadows, such as shadows on a face when a person is standing in shade.
Saturation, Contrast, Gamma, Exposure, Shadows and Highlights allow you to make different adjustments in the color and dark/light relationship of your photo.
Clarity gives you the option to blur the image a bit, for a dramatic effect.
Once all adjustments have been made, you can click on the arrow in the bottom left to return to the list of tools. In the top right, you can either undo the recent changes, or click Save Changes to save all changes made to the image.