Robert Ballard's Bismarck is reprint of the 1990 edition of Discovery of the Bismarck. The only major difference lies in the fact that there are slightly more photos related to subject matter. If you already owned the 1990 edition there is little reason to buy this book unless your old edition is getting worn out.
The book is basically divided into two parts, the first and last parts of the book deals with discovery of the Bismarck, the famed German battleship that had a momentary glory against the British Royal Navy before being sunk by them. I thought the narrative on the ship's discovery and the photographs and illustrations of the wreck was well presented and highly interesting.
The middle section of the book consisted of military history of the Bismarck. The history of the ship, from the beginning to the end is retold here. I thought this was pretty well done and a great introductory material to anyone who wants to read more on the subject.
Overall, this book and the 1990 edition serves it purpose very well and Robert Ballard have done a great job in presenting the Bismarck in her glory as well as in her ruins.