Since before WWI. I'm reading up on that war, and am in the middle of the overview of all naval action in that war (not just the North Sea/Jutland): http://www.amazon.com/gp/product/0870212664/

Quite good, e.g. one Admiral really needed a Nelson era signal in one early battle, a not uncommon way to deal with submarines was to run over them. And it really brings home how it was a "World War" (although not called that at the time).

I would add that anything that stimulates interest in WWI is good. While WWII and on are more studied, so much of the 20th century is founded in WWI and its genesis I think its essential to get a good understanding of it (if you're into this sort of thing, of course).

Somewhat coincidentally, I recently started my serious, systematic study of it. E.g. right now I'm working through the English book that's the single, detailed history of all naval actions in what was truly a world war: http://www.amazon.com/gp/product/0870212664/