The pre-xM original BeagleBoard had a consistent link of where to find the latest version of the System Reference Manual at http://beagleboard.org/static/BBSRM_latest.pdf.
Originally, I thought that I’d eventually replace that link with a System Reference Manual that covered both the pre-xM boards and the xM boards, but that no longer seems likely to happen, so I’d like to get distributors and others to simply refer to the BeagleBoard-xM System Reference manual at http://beagleboard.org/static/BBxMSRM_latest.pdf.
Hope you find this helpful!