What is a "sprite"?
If in the book they are talking about image sprites for the web they are images that contain multiple images, like for instance a series of icons to be used in a navigation bar. You would have one http request for the "sprite" and use CSS to display the icon needed. You can have sprites that contain all of the images for a given page if you wanted. The main reason for using them is to lower the amount of files to be downloaded for the website or webpage.
Hope this helped:)