The Book editor may show a grid and/or guides to help you align text boxes and images on book pages.
To show the grid, select the Show Grid option on the product ribbon.
To show guides, select the Show Guides option on the product ribbon.
Note: You can also drag the guides to place them where you need them.
Snapping to Guides
The Grids and Guides tool helps to precise the alignment. With Guides enabled, text boxes and imported images will fit inside the space between the guides. If you use guides to align objects on page, you may want to make objects snap to guides. To enable snapping, select the Snap to Guides checkbox on the toolbar.
When selected, the guides will act like magnets attracting objects as you move them with your mouse.