'The Ghost in the Machine' is Koestler's classic work on the human mind and human society. Despite taking Ryle's jibe at dualism for its title, the book isn't specifically on the philosophy of mind. Rather it criticises the whole area of mechanistic models of humans and human behaviour. Koestler packs a great deal of discussion into the book, looking at the nature of humans on all levels from neurons to global society. Koestler was going against the tide of the new thinking in biology, and I'm not convinced that the book heralded a revolution in the life sciences, as some would claim. However the arguments are skillfully put and this overview of the human condition is well worth reading.
The book starts with a criticism of behaviourism in philosophy - already on the wane, but Koestler wanted to warn against a resurgence. He goes on to examine heirarchical organization, and how it applies to organisms and societies. There are several chapters looking at evolution and at the development of living things. The 'ghost in the machine' part follows, looking at the human mind and whether it can be treated in a mechanistic way - Koestler thinks not, but his point of view isn't just simple dualism. The book concludes in a depressing fashion, with the idea that an urge to self destruction is built into humankind, and that possibly the only way of dealing with this is the 'Brave New World' idea of suppression via drugs.