Wren, no doubt, has thoroughly researched his topic. He has lived with these operas and knows them intimately as anyone could. One cannot help but admire the depth of analysis given to several of the Savoy operas. I was particularly impressed by his analysis of the subtle flower imagery running thru Ruddigore, and his work on Gilbert's poetical schemes. His musical analysis falls short most of the time, and betrays a lack of understanding regarding Sullivan's approach (or indeed, theatrical music in general). His attacks do not suffer from the German 19th century-influenced prejudices which affected Gervase Hughes' study of Sullivan's music but often miscontrues meaning and in some cases, misses the joke entirely (yes, Sullivan could joke right along with Gilbert; Gilbert in fact famously remarked "I never had to explain a joke to Sullivan").
Wren decries the simple and lovely strophic setting of Patience's air "Love is a plaintive song," complaining (like some other authors) that Sullivan failed to capture the lyric. Not true. The simple, largely diatonic setting exquistely captures lyric and character's frame of mind-- Patience is simple and straightforward, she is unhappy, confused, and parroting what Angela has led her to think love should be; a full out, through-composed dark aria would be uncalled for, and unconvincing from this character. In the same opera, when we meet Grosvenor in Act I, the music that accompanies his entrance is nearly Wagnerian in its dense chromatic wandering, admirably suited to a trendy, artistic young man of the time. Later, when Grosvenor emerges as "An Everyday Young Man," he sings a song that is literally reduced to two chords-- a subtle joke to be sure, but the humor is evident. Wren applauds Sullivan for relying less on recit in later operas, taking it's lack of use as evidence of compositional maturity (a prejudice similar to Gervase's, in that through-composed opera was somehow philsophically preferable to the recit-aria Italian format) when in fact, Sullivan uses recit only where it serves his purpose-- and doesn't when it is not called for. In Ruddigore, Dame Hannah's first song is interupted by a recit like passage-- to call attention to the lines she utters, when Sullivan could have every easily continued in the verse-form. This surprising and effective touch is evidence of compositional maturity, not a lapse into formula by an undeveloped talent.
The bibliography at the end is thorough but also full of editorializations on each book mentioned, which could have been informative but instead are too subjective.
All this being said, the book is worth reading and is an outstanding resource for the informed analyses of the lyric and historical backgrounds, but it should be complemented by other books on the Savoy operas and certainly an acquaintance with good productions.