What is the first realistic poetry collection?

It's the Book of Songs.

The Book of Songs is the beginning of ancient Chinese poetry and the earliest collection of poems, among which six poems are Sheng poems, that is, they have only titles and no content, and are called Six Sheng Poems. It reflects the social outlook of about 500 years from the beginning of the Zhou Dynasty to the end of the Zhou Dynasty. The Book of Songs is divided into three parts: style, elegance and ode. "Wind" is a ballad of Zhou Dynasty. Elegance is the solemn and elegant music of Zhou people; Ode is a musical song used in the ancestral temple of Zhou and aristocratic families.