The National Research Council (part of the National Academies) has released three new reports as part of the series called America's Climate Choices. The three reports are Advancing the Science of Climate Change,Limiting the Magnitude of Climate Change, and Adapting to the Impacts of Climate Change.
I believe that these should be treated as authoritative resources on this issue. The National Academies are institutions composed of America's most accomplished researchers, with a long history of excellence and political independence (though its members probably do have a leftish bias). I'm much more comfortable with their conclusions than I am with the U.N.'s ad hoc organizations such as theIntergovernmental Panel on Climate Change. While the IPCC basically agrees with the conclusions put forth by other bodies of experts, the fact that it has a narrow charter from the UN makes me doubt its political independence, both in terms of its agenda and its selection of experts. America is fortunate enough to have its own collection of domestic experts who can report on these topics from an American perspective.
Two more reports are in progress, and these above reports are considered "pre-publication" so there is some restrictions on how you can read them. However, short summary reports (PDFs) are freely available, and the full report can be read on the web for free, or downloaded for only $2.50. I expect that it will be free in the future, as other National Academy reports are.