It depends on what kind of a book you are working on. For books that involve a lot of references (think academic texts), then my preference - based on my personal experience - would be for a laptop. For web-related work, depending on the nature of the work, the iPad may work. For other kinds of more passive work - like reading, annotating, making notes etc - the iPad is excellent. Note though, what I mention is most obviously coloured by my personal experience using a laptop and the iPad.