Preloading in the background

Most of you will know this beautiful site:

When you click on 1 of the books, underneath it you can see how the ‘upcoming pages’ are loaded while you click through the first couple of pages. Now my question is this: how can I load stuff that’s ‘a couple of blocks down’ while I go through the openig part of a site? I know you can simply preload an entire website, or load content dynamically at the click of a button, but how do I get content to load ‘in the background’ while I already see the first content? Cheers!!