Watchmen has sold around 750,000 copies since it was first published in 1986. Following its appearance in Time magazine's Top 100 English Novels since 1923 and the release of the Watchmen movie trailer, it has been racking up sales at an astonishing rate these past few months. It was the highest-selling graphic novel in the USA in 2008 and DC—Brian Hibbs revealing recently that Bookscan figures put the 2008 US sales at 308,396 copies (and that's probably a low figure as Bookscan does not cover all outlets).
If you've never read the book, now's your chance—you won't be disappointed. Here's a quick rundown and Amazon links to various editions of the original graphic novel and a number of movie tie-in books, plus the first of what will probably be a ton of academic studies of the series.