Books online has, as promised, been updated and available for download from Microsoft. However there are still no pages covering the standard errors in SQL Server as there were in SQL Server 2000 BOL. Look in 2000 BOL for "220" and you get a nice description of error 220 "Arithmetic overflow error for data type %ls, value = %ld." however look in 2005 and nothing. I've been told they will be available in a central place online, but I still can't find anything.
( I mentioned this before http://www.sqljunkies.com/WebLog/simons/archive/2005/11/05/17302.aspx but still no luck)
Thanks to Euan G for the notification about the update http://blogs.msdn.com/euanga/archive/2005/12/07/500750.aspx.