I'm quite taken with this volume. Kline's book runs through all of the G&S operas, offering a hands-on approach to directing the works taken from twenty years of experience.
For a start, this is one of the earliest books to deal with Thespis, The Grand Duke and Utopia Limited. These were essentially not seen in the UK for the first seventy-five years of the century, as the D'Oyly Carte Company - who held the copyright - did not include them in the repertory (Thespis, of course, has other issues). They have only crept in properly in recent years. But, being American, Kline was able to draw from a different tradition where all of the works were generally performed.
Each opera has an overview of characters and casting requirements, a step-by-step plot which includes theatrical notes, and a separate section on staging and costuming. Around this, Kline includes chapters offering overall guidance on forming a G&S company, running rehearsals and scheduling, blocking numbers with multiple singers, and appendices of some cut numbers with the vocal score.
Of course fifty years have taken their toll, as even amateur directors may have access to a far greater range of technical options for their works. The approach in the 1970s was still broadly traditional to the works - not that there's anything wrong with that - but, again, Kline's nationality is a strength here as he shows himself more susceptible to directors messing with the "tradition" should they need to. Still, directors should not feel constrained by the discussion here in that regard.
Ultimately, I think there is still much of value here for anyone wishing to produce the works. There are only a few books on the production side of the works, so they should be cherished.