Panshin, Alexei. Heinlein in Dimension: A Critical Analysis. Chicago: Advent Publishers, 1968. An overview of Heinlein's works to the time of publication. An accessible critique for young readers.

Patrouch, Joseph. "Robert A. Heinlein." In Twentieth-Century American Science Fiction Writers, edited by David Cowart and Thomas L. Wymer. Detroit: Gale Research, 1981. A good introduction to the range and scope of Heinlein's work.

Slusser, George Edgar. The Classic Years of Robert A. Heinlein. San Bernardino, CA: Borgo Press, 1977. A good source of some of Heinlein's most enduring themes, particularly applicable to Red Planet and Stranger in a Strange Land.

Robert A. Heinlein: Stranger in His Own Land. San Bernardino, CA: Borgo Press, 1976. A slightly scholarly but very good 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...