The following are useful websites (in no particular order)
The BBC website http://www.bbc.co.uk/news/health-12722435 contains a relatively good basic source of information on radiation risks. The word ‘relatively’ is used because the website claims that the “lowest level at which any … increase in cancer risk is clearly evident” is 100 mSv which is incorrect. There is a great deal of evidence of adverse effects well below this level.
http://books.nap.edu/books/030909156X/html/R1.html to download US BEIR VII
http://www.criirad.org/ (in French)
http://hpschapters.org/northcarolina/NSDS/210PoPDF.pdf for data on Po-210