This one was hard to get through for two reasons: one, because of the wealth of scientific data combined with my lack of science knowledge made for tedious reading, and two, because of the sense of powerlessness one gets upon learning about this enormous and tremendously important issue that is largely unreported in news media or understood by the general public.
In short, pesticide companies like Bayer and BASF are spraying flowering crops and trees with neurotoxic chemicals like imidacloprid, fipronil, thiamethoxam, and clothianidin. Neurotoxins (like DDT) are designed to take out an insect starting with its nervous system, and they are very potent, requiring only a small dose to be effective. These long-lasting toxins remain in the environment for years, and the combined effect of honeybees feeding on treated crops and feeding on plants grown in ground that was treated years ago is taking its toll. The bees take this toxic pollen back to the hive, where they store it until the fall (in the US). Once the bees eat this contaminated nectar, their immune systems become so compromised that if the neurotoxins don't kill the bees, then varroa mites or exposure to additional pesticides or malnutritious monocrop diet (a la commercial bees, who are trucked in to feed on one crop at a time) will do them in.
[If people more science-minded than I find an error with this last paragraph, let me know: I will freely admit that I may be oversimplifying/misstating in trying to create sense out of a complicated situation.]
If you care about the effect of humans on the environment, you've considered raising honeybees, or you just have a yen for science, I highly recommend this book. You won't regret the knowledge you will have gained.