My reading pattern on subs has coincided with the game I'm playing. For SH3, there's a wealth of material on the web that kept me reading for hours.
Now that I've just started playing SH4, I need to switch over to the Silent Service. While there are some very good sites out there, they don't provide the same vast wealth of uboat material available online.
I just ordered
US Submarines 1941-45 for elementary quick reading reference. I also ordered the DVD
Submarines: The Silent Service in World War II.
Now, if I could only stop humming the SH3 theme song when I'm playing SH4.

It's tough to switch over!