Hi Tony,
There are folks more qualified than I to respond here, but my take on this is along the following lines:
As an analogy, your question is rather like asking what is the best causation study for smoking and lung cancer after the 1964 Surgeon General's report. To be sure, the science has progressed in many ways since then, but once something has been conclusively demonstrated, there's not really such a strong scientific need to demonstrate it again.
But there is clearly social and public health value in re-making the case from time-to-time. Unfortunately, the resources needed to go through the kind of major exercise that the National Academy of Sciences did under an EPA grant 20 years ago are not so readily come by, so I can't point to any single new review that has the singular weight that BEIR VI did in its time.
Of course, you are aware of EPA's 2003 Assessment of Risks from Radon in Homes. And US Surgeon General Carmona issued a prominent National Health Advisory on radon in 2005. In 2006, the NAS also put out a more general review of Health Risks from Exposure to Low Levels of Ionizing Radiation (BEIR VII), but radon was not a big focus of that report.
International publications such as the United Nations Committee on the Effects of Atomic Radiation (UNSCEAR) series of reports on "Effects of ionizing radiation" (including ... 2006, 2008, 2012, 2013, 2016) keep track of current science but these are not on the radar for most, and their content is beyond most readerships. The World Health Organization's 2009 Handbook on Indoor Radon is probably the best known international document summarizing the science and policy perspectives on radon for a more lay readership, but even then you're quite likely to encounter a lack of awareness about this publication. Other documents you could refer people to include ICRP's Publication 115 (2010) that adjusts the risk coefficient upward, and of course the Health Physics Society's 2009 revised (be sure you aren't looking at the 1990 version) position statement on radon.
Nevertheless, for purposes of having material that summarizes the science cogently, I do routinely suggest that people review the materials (available on www.radonleaders.org ) from Dr. Bill Field and Dr. Jay Lubin--their papers and presentation before the President's Cancer Panel (2008). Anyone who wants to read more, such as the analyses on the pooled radon epidemiological studies, can follow the references therein.