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If you go to the conservation track in the UCSC browser (http://genome-euro.ucsc.edu/cgi-bin/hgTrackUi?hgsid=193841314&c=chr21&g=cons46way) the key publication for phastCons appears to be Siepel et al. 2005. Apart from that I cannot find anything else.
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On the left you have links to PhastCons and PhyloP manuals, at the end of each manual there is a list of relevant literature