I have read all biographies of Catherine The Great I could find and they all have something in them that is unique. Robert K. Massey's Catherine includes extensive excerpts from her memoirs, letters, and other primary sources--adding authentic voices to the narrative. Massey calls his biography "a portrait of a woman" and he is consistent in showing Catherine's feminine side.
I found his discussion of historical background very illuminating--he cleared a few mysteries for me--however I found that the last years of her life did not get as thorough a treatment as earlier periods. I was looking forward to his take on Platon Zubov, Catherine's last favourite, but got little there beyond a tantalizing statement that Zubov was different than her other lovers. Her earlier liaisons have been discussed with more details and are far more developed.
With all these flaws, the book is splendid and a true complement to the existing biographies of this enigmatic, fascinating, and powerful woman. I heartily recommend it.