One of the things that Microsoft committed to back in the SQL Server 7 to 2005 time frame was to do a much better job of updating the SQL Server documentation. They originally commited to a quarterly update, but for the most part it has been two or three times a year. Far better than just shipping a static set of documentation upon release!
One of the reasons for the regular and frequent updates was to communicate current security information about threats to SQL Server and what you can do, as an admin or developer, to keep your installation of SQL Server secure.
So you should definitely update you copy of Books Online every time there's an update. A great place to get information about updates is on the SQL Server User Education team's blog (they're the people who write the documentation). They usually only post when there is some new release, so adding it to your blog reader won't result in undue amounts of traffic.
As I type this, Refresh 9 of SQL Server 2008 BOL is online. At the moment, it's only available on MSDN online. The installation files for a local copy usually lag by a month or two. Which means that soon you should be able to download it for local use.
Happy reading!
Don