Microsoft has come out with the documentation to SQL Server 2005 Express here.
Here is a description on the Microsoft Website-
Download samples for SQL Server 2005 Beta 2 and product documentation for SQL Server 2005 Express Edition. The SQL Server samples available for download can be used with SQL Server 2005 Express Edition. SQL Server 2005 Abridged Books Online for SQL Express includes SQL Server 2005 documentation that also applies to SQL Server Express (the successor to MSDE 2000).
This documentation includes:
- SQL Server installation
- SQL Server database engine
- SQL Server replication
- SQL Server and the Transact-SQL language
- SQL Server programming interfaces
Download the documentation here.