With regards to Sirhan Sirhan and the conspiracy against RFK, I highly recommend David Scheim's aforementioned Contract on America, which explores both the Kennedy murders, and Dr. Martin Luther King's as well, if I remember correctly (I don't own this book, but my local library has a copy which I'm wearing out).

Scheim, if I recall, does provide evidence that the shooter was one of the guards/officers behind RFK at the time. He has pictures to aid the theories, which help immensely. I highly recommend this book.