Yoy I would recommend reading ONLY part one and two. Three was made up with pieces for a Doctor Who series that were never used, and does not feel like a real Hitchhikers' book. I did not like it. Four and five, I and don't know yet, am reading them. But they can never be better than one and two. And yes, it is necessary to read the first book first. If you wish, you can read only the first one.