I've seen other people mention being unable to get it to re-dock (on certain versions of Windows, I believe), as well, so it may not be as simple as it sounds.
If double-clicking the title bar of the Book Browser fails to re-dock it, I've heard of others having to resort to deleting the sigil.ini file to fix it. Just make sure there's no keyboard shortcuts or other customizations you can't bear to lose before deleting.