The subject being Napoleon's appointed marshals, his army commanders. Mr Delderfield appears to be a fan rather than a student, which is OK, and his book is mostly a listing of well-known facts which you can find in other, better works. If you can get a cheap copy it will serve as a general introduction to the marshalate, but it contains no revelations other than the fact that RFD obviously liked some of the marshals much better than others. Ney appears in his red-haired, bravest of the brave personna, with little reference to his strategic incompetence and, of course, Marmont the traitor is suitably vilified for surrendering Paris and his Corps to the Allies in 1814, ignoring the fact that he was acting under instruction from Joseph Bonaparte and the French government.
And so on - let's not get into all that; as a taster this is reasonable, but the works of David Chandler, John Elting and Kevin Kline are factually more competent, better balanced and - well - better written, really.