If you're using the local client of Books Online, you can get the version released in February of 2007 here:
http://www.microsoft.com/technet/prodtechnol/sql/2005/downloads/books.mspx
If you're using the Web to read Books Online, it is automatically updated. You can read that here:
http://msdn2.microsoft.com/en-us/library/ms130214.aspx
There is also a wealth of other information here:
http://www.microsoft.com/technet/prodtechnol/sql/2005/learning/default.mspx
