In this book, Clegg examines seven threats that could kill many or even all of us such as climate change, nuclear weapons, and nanobots. He provides background and history, describes the dangers, and assesses their likelihood. Far from being an alarmist, he dismisses most of them, at least for the near future.
Sometimes the author gets sidetracked on rants, such as his take-down of Dan Brown's Angels & Demons (another example is his rant against the UK's Soil Association for its ban on nanoparticles). It doesn't mean anything to me since I never read the book or saw the movie, and besides, the type of person who would believe the science in a Dan Brown book wouldn't read a book like this one (though that person might own a Def Leppard album or two).
Since entire books have been written about each of these topics, Armageddon Science isn't the deepest discussion of them. Still, it is an entertaining and informative read.
Copyediting note: Clegg misspells tokamak as "tokomak", and boy, do I feel like a smart science guy for catching that one (I read a book about fusion a couple years ago).