Pen & Sword is another publisher with whom I have a mixed attitude towards, as for every solid work there seems to be two pot-boilers. This book is one of the good ones, as Harrel examines the roots of Russo-Soviet cavalry doctrine, provides a close examination of how this played out in the Great Patriotic War, how this doctrine carried forward post-1945, and the ultimate demobilization of the Soviet horse units.
As for why the Soviet cavalry units retained relevance, Harrel emphasizes two main points. One being that there was a lot of terrain in the Soviet Union which was impassible to armored units, but where men on horses could get through. Two, the leadership of the cavalry suffered less from the decimation of the Great Purge, and remained a kernel of professionalism in the Red Army.
That said, this is very much staff-school history, and very focused on the fine details of tactical-operational deployment. This is not the first book you want to read about the Russo-Soviet military experience; it might not even be the fifteenth.
As for other issues, for as much work that seems to have been put into this book, there are a jarring number of typos. Also, early in the book, there was one point where Harrel touched on the differences between the Russian and German cavalry experiences, and I read it a few times and I'm not sure it really made sense. However, to be fair, this is not a book about the "Reichswehr" of the Weimar Republic.