I've just installed the January 2009 refresh for SQL Server 2008's Books Online. I've only glanced at what's different (http://msdn.microsoft.com/en-us/library/dd408738.aspx), but I just love the fact that SQL Books Online is a work in progress. Books Online lets you provide feedback on any page, and that feedback often makes it into an upcoming refresh.
Of course, the whole of Books Online is available online, but I like to have a local copy, which gives me better access to the Index page. Now, if they gave me an option to run it in a hybrid mode, querying online for updated data, then that could be really neat too — and avoid the need to download the refreshes. But for the time being, get yourself along to http://www.microsoft.com/downloads/details.aspx?FamilyId=765433F7-0983-4D7A-B628-0A98145BCB97&displaylang=en and grab a copy.