Sidebar at bottom of page (yes, that again)

My Now Reading plugin library page doesn’t want to play nicely with my sidebar – see https://www.conflictoflaws.net/index.php?now_reading_library=true

Any ideas what I should be closing/inserting or something else in order to make them kiss and make up?

(I realise is fails validation horribly, but this seems to be at least some of the plugin’s fault and I’m not about to cure all the errors; if you know which one is causing the problem, though, that would be lovely.)

Many thanks.