Cowart, David, and Thomas L. Wymer, eds. "Robert A. Heinlein." In Twentieth Century American Science Fiction Writers. Detroit: Gale Research, 1981.

A good introduction to the range and scope of Heinlein's work.

Panshin, Alexei. Heinlein in Dimension: A Critical Analysis. Chicago: Advent Publishers, 1968. An overview of Heinlein's works, this is an accessible critique for young readers.

Slusser, George Edgar. The Classic Years of Robert A. Heinlein. San Bernardino, CA: Borgo Press, 1977. Examines some of Heinlein's most enduring themes. Many of the observations in this book can be applied to the concerns of Farmer in the Sky.

. Robert A. Heinlein: Stranger in His Own Land. San Bernardino, CA: Borgo Press, 1976. A useful, scholarly assessment of Heinlein's more modern works.

Williamson, Jack. "Youth Against Space: Heinlein's Juveniles Revisited."

In Robert A. Heinlein, edited by Joseph D. Olander and Martin Harry Greenberg. New York: Taplinger, 1978. A good summary...