I'm a drama student at uni and found this book absolutely wonderful. For essays it's incredibly helpful because it goes into a lot of detail about Stoppard's work and so you can quote away to your hearts content.
A problem I have often found with drama 'reference books' in the past is that they can be very dry and difficult to read but I would recommend this to absolutely anyone but this was written in an understandable way.. It's a fascinating read and should be read by anyone with an interest in theatre, whether it's professional or recreational.