To straighten an image in a photo book you have to first select the image by clicking on it. Once you've selected the image you'll see a number of icons appear on and around the edges of the photo. In the upper left corner of the photo you'll see a circular arrow icon (See below). Clicking on this icon (and keeping the mouse button pressed) will launch the straighten tool.
Once in the straighten tool, you can drag the mouse pointer to the left or right in order to tilt the image (within the photo placeholder) in that direction. As you tilt the photo, a tooltip will be displayed (see below) that shows the image's angle of rotation. Once the photo is at the angle you want, release the mouse button to keep it at that angle.