Hippolytus by W. S. Barrett (1964)

Published by Clarendon in 1964, this is still regarded as the most important scholarly commentary on the play. It discusses textual and interpretive issues.

Euripides: Hippolytus by Sophie Mills (2002)

This is a useful general overview of the play that discusses the ways it has been interpreted by scholars. It was published by Bristol Classical Press in 2002 as part of the Duckworth Companions to Greek and Roman Tragedy series.