Panshin, Alexei. Heinlein in Dimension: A Critical Analysis. Chicago: Advent Publishers, 1968. An overview of Heinlein's works. 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 that analyzes some of Heinlein's most enduring themes. Particularly applicable to Red Planet.

Robert A. Heinlein: Stranger in His Own Land. San Bernardino, CA: Borgo Press, 1976. A slightly scholarly, but thorough assessment of Heinlein's work.

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 of most of Heinlein's books for young adults...