Off the shelf normally mean ready made products that you buy off the shelf, products that are not custom made. Many times it refers to a software product that is made for the general public. So what could an off the shelf chess game mean (answer later)?  

 A chess "book move" has a very simple definition. Simply put it is an opening move in chess that have been studied and analyze for hundreds of years. These moves have been used and accepted by some of the great Grandmaster Chess champions and players. You could say it an off the shelf move or product.  

Moving on a little, let compare an off the shelf product with a "book move". Let think of a jar of jelly as an off the shelf product. First of all a jar of jelly is a product for the general public. It been studied and analyze for hundred plus years, on how to store and display it.  Now when you see a jar of jelly in the grocery store, in a glass jar that is the results of many year of study just like a chess "book move". It normally in a glass jars with a removable top because it been determined it is the best way to store, preserve and display it. This is the same as a "book move", analyzes, tested, accepted and available for the general public.  

In chess some opening moves are referred to as "book moves".  These moves have been studies for hundreds of years. You could consider these opening series of chess moves as an off the shelf product. It’s available for the general public.  

Last but not lease let talk about a 'book game". A "'book game" is a game that a chess player has played many times. Of course the player has not played it for hundreds of years because most don’t live that long.  

A chess player "book game" has been tested and played many time by the player.  When you enter your own "book game" it requires very little thought, you just follow through with a game that you played many time before.  Almost a one hundred percent win.  

@rodrook