Staring the obvious here but SQL Server books on-line (BOL) is called that for a reason, it’s constantly being updated …on-line.
I mention this because every now and then a new version becomes available for download, and in fact there was an update for SQL Server 2008 BOL on 30 January. Keeping up to date is important at the moment as BOL still seems to be catching up with what’s in the SQL Server 2008, a good example of this is the section on audit which is now much more helpful.
You may prefer to always use BOL on-line at TechNet here, but I find search easier on the local copy, for example on TechNet you end up searching all of Technet, while on a local copy you can refine your search to a just a part of BOL if you want to e.g. T-SQL, reporting services etc. and surfing for an answer is not always possible at some sights so you need your local copy.