Effects of Nuclear Weapons published by the US Government is by far the most authoritive official unclassified
source. Most other sources are based on compilations from this book.
Though out-of-print, different editions (1962, 1964, 1977) are still available from
Amazon, eBay etc.
Several basic chapters are available on-line
Courtesy Trinity Atomic Web Site
These 2 documents were issued by the Canadian government.
Both are based on "Effects of Nuclear Weapons".
Courtesy Bruce Beach, Radiological Scientific Officer, Canada.