The full title is: Shadow Divers: The True Adventure of Two Americans Who Risked Everything to Solve One of the Last Mysteries of World War II, and it's by Robert Kurson.
What it's about is a little more complicated.
In 1991, two deep-sea divers found a mysterious wreck 230 feet below the surface (right around the limits of technical deep-sea diving at the time.)
The mystery comes in trying to figure out what the wreck is - and more importantly, what it's doing there. Identifying it in general terms was pretty easy - it's a World War II U-Boat. But there's a catch: so far as the existing records said, no U-Boat should have sunk in that location.
The book tells the story of both the diving investigation (along with the risks of doing very deep dives and figuring out what combination of gases in the dive tanks worked reliably) and of the research once they'd found identifying information on the wreck, which culminates in them identifying some of the German sailors lost on the wreck and speaking with their surviving families. It's a great example, for this reason, of how the research and learning we do connects with real people and their hopes, dreams, and tragedies.
The book has come under some criticism for depiction of various events, but despite that remains an interesting read, with some fascinating side trips into related topics. Well worth a read by anyone interested in high-risk sports, the ocean, WWII history, or the joys of research.