According to this, there was a Panda update on Sept 18, so I suspect that's what hit your site. Panda mainly targets the content of your website - my guess would be that your site was penalized because it has a lot of "thin content" pages. In other words, all your book pages have very little (no?) unique textual content.
FYI, I would say your US site is also in danger of being penalized by Panda and/or Penguin. I see that over 1/3 of linking root domains link to you with the anchor text "<a class="clickable title link-pivot" title="See top linking pages that use this anchor text">buy books online". Over-use of keyword anchor text like that is strongly correlated with getting a Penguin penalty.</a>