These are the manual and articles that are officially part of the AngelScript SDK.
Application Writer's manual
This is the reference manual that describes the programming library and the script language.
This is the change list for all the releases of the library.
I haven't prepared any offline documentation besides the html documentation that is included in the SDK and also available online. If you prefer to use offline documentation then you can easily generate it yourself from the Doxygen source available in the SVN repository.
Use your favorite SVN client to download it, or else go to the SourceForge.net project page and download a tar ball.