Please follow the given steps to use the zoom feature on the eBook:
You can zoom in into a paragraph or a diagram on the eBook
Use your fingers and pinch-to-zoo gesture on the touchpad of your MacBook or Laptop
Or, if you are using Magic Mouse you can simply double click
Or, you can use the following key combination on your keyboard
On Mac : Press 'Command' and '='
On Desktop PC : Press 'Ctrl' and '='
Please watch the given video for example