You're not the first and certainly won't be the last to fall prey to the 'book project'. Fact is, if he knew what he was doing he would be pitching his idea to a publisher first, but really his story is crap so a publisher wouldn't touch it. Secondly he couldn't afford a well established and published artist (no offence but we all have to start somewhere) so that's where he feels you will do it for the love. You are very talented, take it as a lesson (same happened to me many times) and be very upfront about your charges next time and even take a first instalment (maybe half) to make the client commit. Value your work and good clients will come. All the best