Darwin is well known for his works on evolution, in particular the 'Origin of Species'. However he worked in many other areas of biology, and it is sometimes said that even without his work on evolution he would have been considered one of the eminent biologists of the 19th century. In this book Mark Ridley has put together a selection of chapters from the works of Darwin, giving an indication of the breadth of his work. Ridley has chosen well, often helping to illustrate something of Darwin's mode of thought. Naturally there are chapters from the books on evolution, but there are also ones on his early work on the formation of coral reefs, and his later work on the action of earthworms.
Although Darwin is recognised as a skilled writer, and certainly aimed at the 'popular science' market, the length of time since he wrote means that reading his books can sometimes be something of a struggle - it takes some time to get used to his style of writing. Hence Ridley's selection is an excellent idea, giving the reader a gentle introduction to his books. If you are interested in reading more then you might like to know that many of Darwin's works can be read online at the Gutenberg project