Is SQL Server 2005 Books Online updated in September 2007? Yes it is!
Well this is confusing me, this DownloadLink refers SQL Server 2005 Books Online as May 2007 update that can be installed on the machine. But when you search for any content on MSDN library for SQL Server it refers "SQL Server 2005 Books Online (September 2007)" on the sections (majority of them I have seen).
I'm aware that many times if the SQL Server component is updated several times a year say with hotfix or service pack, SQL Development team tends to release a README file or even updated books online which is famously called "web refresh".
So now the question if the majority of the BOL content is updated in September 2007 why not release a new download to update the local copies of the BOL.
I will update back here once I get a proper response from SQL Development team.
29th Septemer 2007;
Here is the Update