An U.K. company, Seaforth Publishing, which I gather to be an imprint of the better-known Pen & Sword publishing company, has a huge number of coupon codes pinned on Pinterest. And many of them are very good ones. Here's the URL of the Pinterest webpage where you can find them:
www.pinterest.com/pin/521502831826933931.
Seaforth Publishing, appears to produce only titles related to--you guessed it--ocean-related stuff, in particular ships used in the military. Pen & Sword's focus is quite a bit larger, taking in history in general, but largely of military ships.
One enormous caveat: pins with expired coupon codes apparently don't get taken down (this may be a problem with Pinterest and sites similar to Pinterest, I just don't know). This means that you either need to check separately each pin showing a discount code or link to a discount, or if you computer-savvy enough to do this, to search the html source page. If trying the latter, you might try searching for words like "(discount) code," "(book) code," and maybe some others.