Through the Looking Glass is a great website on early submarine history from 1900 to 1940.  There is a lot of hard to find material here, many books worth, presented as collections of photographs and illustrations.  Here's the disclaimer from the site:
Be Warned! These pages now contain over 1300 photos and each page may take some time to load.

This warning gives you an idea of how good the site is.  And it gives you a reason to order that DSL or broadband cable connection for faster Internet access.
  Through the Looking Glass - Submarine Photo Essays [1]