The book chronicles Darwin's experiments with various carnivorous plants, in which he carefully studied their feeding mechanisms. Darwin tried several methods to stimulate the plants into activating their trap mechanisms, including feeding them meat and glass, blowing on them and prodding them with hair. He found that only the movement of an animal would cause the plants to react, and concluded that this was an evolutionary adaptation to conserve energy for prey and to ignore stimuli that were not likely to be nutritious.He also discovered that while some plants have distinct trap-like structures, others produce sticky fluids to ensnare their prey and concluded that this was an example of natural selection pressure resulting in various methods for food capture.I haven't had time to read the entire book, but from my quick scan of it at the university library, I found it quite interesting. The book is scientific, but easiliy understandable to the layman, and goes through Darwin's experiments testing the mechanisms of several carnivorous plant genera. In the drosera section, he tests the effect of different substances and conditions which stimulate a leaf reflex, even testing various snake poisons.
This will certainly be an interesting read for those who are interested in the scientific side of the plants. Keep in mind that the book was written in the 19th century, so advancements in understanding may have been made since.
For those who don't have access to a hard copy, here's the online transcript of the book provided by Darwin Online