Start with high-confidence theater books, then narrow by subgenre or adjacent topics.
Britain and Foreign Affairs 1815-1885
John Lowe
Thinking Like a Director
Michael Bloom
On Directing
Harold Clurman
As You Like It
William Shakespeare
Measure for Measure
The Two Noble Kinsmen
Cymbeline
King John
King Richard III
The Comedy of Errors
Notes on Directing
Frank Hauser, Russell Reich
Midsummer Night's Dream