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 Edit/Crop Photo button on the left. From there, click on Adjust.
Here you'll see multiple options. The first of 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 and Contrast are next. These are pretty self explanatory, simply move the sliders to adjuts the image to your liking.
Clarity gives you the option to blur the image a bit, for a dramatic effect.
Exposure is another way of fine tuning the brightness/darkness of the image.
Shadows and Highlights are last. 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.