The Calvins are a group of researchers hired by the Tet Corporation. Their job is to scour the works of Stephen King for references to real events in the Keystone World, and connections to the Dark Tower. Their purpose in doing so was to arm themselves with knowledge to thwart the efforts of the Sombra Corporation and to assist Roland Deschain in defending the Dark Tower.
They are named after Calvin Tower. They named themselves this in honor of Tower's role in founding the Tet Corporation and his love of books.
The Calvins suggested that Moses Carver should give Roland the book, Insomnia, if he should ever meet Roland. Roland disagreed with the Calvins that Insomnia would help him and he discarded it shortly after its reception. The book felt tricky and subtly wrong to him.