Before I'll make some critical notions on this book I have to confess how much I enjoyed reading this book. This book, like the other books by Taruskin is academic literature at its best. Not only profound, detailled and knowledgable but also driven by curiosity and passionate interest in his subject. And I wouldn't even consider my objections against many of his assumptions as something to be put against this book. I prefer being confronted with some controversial theories to boring middle of the road literature.
Already the title of the introduction "Who Speaks for Musorgsky?" made me a little resentful and is somehow symptomatic for the case Musorgsky. Why is it that everybody feels invited to either speak for him or school him like a dependent boy? Balakirev, Cui and Rimsky did, and even his friends and supporters Stasov and Kutuzov loved to do it. Taruskin decided that Kutuzov speaks more for Musorgsky than Stasov since this fits better to his revisionist thesis. The truth is that they all first speak for themselves. Stasov projects his revolutionary ideas on him like Kutuzov his reactionary aristocratic ideals. Cui wants to demonstrate his intellectual and Rinsky his technical superiority. And of course the soviet propaganda used him for their prposesas well. I would say that even the Musorgsky of the letters is not always speaking for Musorgsky since his hypersensitive and conflict avoiding character often made him write rather what his addresser wanted to hear than what he really thought. The only thing that speaks truthfully for Musorgsky is his music.
The underlying thesis of this book or at least some chapters (the book is a selection of essays written at different times and occasions) is, as already hinted in the introduction, that Musorgsky was only a revolutionary in the early years of the "Marriage" and the "Ur-Boris" but became more and more conventional not to say reactionary already with the revision of Boris. I completely agree with the observation that there is this change from a more recitative dialogue style to more coherent musical entities. But to say one thing is the right way the other the false (compromising) way is a little bit too simple for my taste.
Not only that other composers made this experience as well that the recitative style has its limiations. Wagner turned back after the "Rheingold" which represents his theory of Leitmotivtechnik in the purest form. Debussy wanted to overcome his loose Pelleas style in his planned (but unfortunately never realized) later opera projects. And also Ravels two small operas reflect this experience.
The Ur-Boris definitely has its merits and is by no means defective. However, the additions of the revision process, mainly the "Polish act" and the "Kromy scene", may not be necessary for the plot, but are much more than just giving in to opera conventions. They add new dimensions to the epic feel of the opera. Besides that I would not want to miss the fountain scene, with its burst of emotions one of the magic moments of all opera history.
Especially when it comes to Khovanshchina Taruskin's thesis becomes awkward. There might be more pieces in this opera that look like an aria or a conventional choir scene than in Boris, but if you don't look at this opera with the eyes of a musicologist but experience it as a musical drama you must realize that it is much more radical and experimental (and therefore also much less popular) than Boris Godunov. Its atmosphere is completely different. Boris has this young hero and the element of upheaval and despite some pessimistic undertones it shows (especially in the second act) a relatively intact world while Khovanshchina lacks all idealistic appeal. It is a dark, rotten, pessimistic world close to Shakespeare's late plays (I always felt a strong connection especially to Macbeth). It speaks for Musorgsky's instinkt that he couldn't just continue composing in the good old Boris style. To consider this opera as a work of a reactionary only because there are princes as main characters is not convincing at all, as if the personnel would say anything about quality.
Taruskin sees a conflict in Musorgsky's personality between the progressive "democratic" side influenced by Stasov and the reactionist aristocratic side claimed by Kutusov. I don't really believe that this played a big role since Musorgsky didn't think in this kind of political categories. There are some interesting biographical similarities to Lev Tolstoi which might help understand some aspects of Musorgskys character. Like Tolstoi he was of aristocratic origins and had a happy previleged childhood. Like Tolsoi in his early novels also Musorgsky remembers this time with nostalgic fondness, namely in the song cycle "Nursery" and the second Boris act. Both welcomed the end of serfdom in Russia and went through a phase of idealizing the simple folks but then lost pretty much all illusions. Apart from that they were rather antipodes. Tolstoi was driven by a idealistic spirit and all of his works have some kind of message (although I think that the messages are rather a byproduct and not what makes his novels so great). Musorgsky was a sensualist and explorer of truth. He looks at the bottom of people's souls and created figures with complex characters. He was interested in people no matter of which rank and origin. The song cycle "Without sun" for example, deeply admired by Debussy, which Taruskin considers as self centered aristocratic whiny stuff, are psychograms of depression and decline (here another interesting link to some of Shakespeare's sonnets) and as such first of all human. There is a deep compassion for everybody, not only for the guilt marked Boris but also for the holy fool and even for characters like Andrej and Ivan Chovansky.
With such gifts Musorgsky was a born music dramatist and as a music dramatist he is a genius of the hightest rank. None of the operas by Rimsky and Tchaikovsky can compare with the dramatic power of Musorgsky. I would even go so far to estimate him as a dramatist (not as a composer) higher than Wagner. And even work like "Pictures of an exhibition" I would consider in nuce as a music dramatic work. I don't share the popular belief that if Musorgsky would have achieved more technical skills he would have been a even more important artist. People who think like that don't understand the dynamics of creativity. It might sound strange but Musorgsky's neglect of technical aspects and his tremendous expressive power are undividable.