First of all, this is a long book. A VERY long book. As in, 1130 pages. Every one of which I enjoyed.
So if you're into cryptography, computers, World War II history, submarines, the Philippines, or engineering, you'll love this book. And even if you are only interested in one or two of those things, and know little of the others, you'll still enjoy the story (i.e. I know some WWII history and a little bit about the Phillipines during the Spanish-American War and beyond, but all I know about computers and cryptography is what happened to stick in my mind when people have tried to explain it to me).