If you started working on a layout and realize it isn't to your liking, you can easily start over with the whole book or just the current page. In the toolbar right above your book, click on the last button on the right, the one with an "X" inside the rectangle. This opens a drop down with 4 options:
1) Clear All Images: This will remove all images on the current sheet but the layout remains.
2) Clear All Frames: This will remove all images and the layout on the current sheet. Your sheet will be blank.
3) Delete Sheet: This will delete the current sheet if your photobook has more than the minimum required number of pages.
4) Start Over: This will remove all images and layouts in the entire book. If this option is selected, your entire book will be blank for you to start over.