"The Meaning of Shakespeare" includes a chapter on one of only two of Shakespeare's plays to observe what? 
The Meaning of Shakespeare: The Meaning of Shakespeare (1951) was written by Harold Clarke Goddard. The Meaning of Shakespeare:  A chapter is devoted to each of thirty-seven plays by William Shakespeare, ranging from three pages for "The Comedy of Errors" to over 50 for "Henry V". The Comedy of Errors: The Comedy of Errors is one of William Shakespeare's early plays. The Comedy of Errors:  "The Comedy of Errors" (along with "The Tempest") is one of only two of Shakespeare's plays to observe the Unity of Time (classical unities).
Unity of Time