Why is "Investment" accounted for in Aggregate Demand?

A book store sells 5 books of a specific title. But they also are concerned that the demand seems to be increasing because of the popularity and advertisements. They increase the stock level by 3 to insure that the readers get the books from them when they want them. So the demand in this situation to the publisher is 8 books. 5 to replace stock and 3 to increase the investment in stock to cover anticipated demand. So that is the aggregated demand.