I'm certain that neither one is the book you remember, but Norman Friedman's Submarine Design and Development and David Miller's Modern Submarine Warfare both make for excellent introductions to submarine design, as does John Alden's The Fleet Submarine in the US Navy for fleet boats.
Some others I recommend:
US Submarines Through 1945: An Illustrated Design History, US Submarines Since 1945: An Illustrated History, and British Submarines In Two World Wars by Norman Friedman, The U-boat by Eberhard Rossler, and Cold War Submarines by Norman Polmar.
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