When creating a Monthbook, your camera roll will automatically filter to the month you are working on. If you would like to change that, please follow these instructions to change the filter:
Under the Book that you'd like to add photos to, tap, "Add Photos"
You should see "Camera Roll" in the upper middle and then just to the right, tap "Filter"
From here, you can select the month you would like to see.
Once you have your month and year set, tap "Apply."
You can also check out the videos below to walk you through the process: