The definitive place to start is the Summary for Policy Makers at the start of the IPCC AR5 Working Group 1 on the physical science basis of climate change.
This also acts as a suitable introduction to the massive rest of the physical basis report, and will, together with the contents table for the full report, enable a reader to quickly find the sections of interest that cover greater detail. And to burrow deeper again, if there are specific sections that you'd like to know more about, the thing to do is to follow through and read the references from those sections.
