I think what Boksana misconcluded is about "book".
Actually the last two-three sentences says it all.
Despite the pretensions of certain of his disciples in the United States, the Russian director never intended to formulate a textbook of rigid solutions to acting problems
When argument says "textbook of rigid solution" it does not literally imply text book, what it means is "all the methods used by the director". Argument also talks about "despite the pretensions of his disciples", by this only thing one can infer is that disciples altered the way director used to work. ("Pretensions --> to pretend something, but not to be one in reality"!).
Hence we can safely infer that "director's way of working has lost some of its qualitites"
Hope this helps! let me know if I was not clear in my thought!