I thought the Diesel Maintenence guide was his best one. It did teach me alot about my car and I enjoyed the thorough readings on the "pooper". I do not like how the pictures are all over the place though; he tells you about one thing and shows a picture of another. I also own two other ones which are thumbs down. They are the Vacuum Repair and Instrument Cluster Repair. Nothing too fancy about them and everything is briefly explained as opposed to going into detail.
I am also not a big fan of their service. Whoever answers the e-mails seems to have an attitude like their $hit doesn't stink.
Thanks