I'm finishing the book now. If you enjoy the movie, you won't be able to put the book down, either.
There are, as always, differences, but it's a good adaptation, as they go.
Also, compared to what happened to the Old Man and U-96 in the book and movie, real life played out a bit differntly. The Old Man survived until 1980 and went to to captain other vessels. The U-96 itself was destroyed by air attack, but not until 1945, under a different commander. You can check these and other fun facts out at the wonderful
www.uboat.net . You can easily kill an entire afternoon gettin' your learn on at that site.