To cite the main idea of a work, key your reference to the first element of your works-cited-list entry. This element—usually the author or title—could appear in prose or parentheses. No page number is needed. 

In Fiction without Humanity, Lynn Festa argues that in the late seventeenth century, literature and the arts created the categories of writer and artist. 

or 

A recent book argues that in the late seventeenth century, literature and the arts created the categories of writer and artist (Festa).

Work Cited

Festa, Lynn. Fiction without Humanity: Person, Animal, Thing in Early Enlightenment Culture. U of Pennsylvania P, 2019.