Found a small writeup via Google Books in "101 Wisconsin Unsolved Mysteries"
http://books.google.com/books?id=j9RTJEGSoWIC&pg=PA61&lpg=PA61&dq=Bob+Christian+wisconsin&source=bl&ots=bOrx_4BOPO&sig=CEY7S6E_cxku1VbD4ocSJV_mkkU&hl=en&sa=X&ei=3U4XUIfEBISHrAGupYDQCQ&ved=0CFIQ6AEwBzgK#v=onepage&q=Bob%20Christian%20wisconsin&f=false
Interesting points that we know so far:
Bob left his home at 5:30pm in the family car.
Battery, plate, hubcaps and wheels were stolen.
His hunting equipment was missing.
Hubcaps and wheels found a week later with bullet holes (LE believes they were used for target practice)
The family hired a PI and two psychics and followed many psychic leads.
In 1983, Bob's sister Kathy died in a fire at a bar. Some members of the Ghost Riders motorcycle gang were charged with her murder. LE determined that this was not related to Bob's disappearance.
Timeline:
Friday Sept 16. 1977:5:30 pm Bob leaves home (Reedsburg)
after 5:30 pm Mom sees Bob around area
8:00 pm witness sighting around Durwards Glen, Baraboo, WI
9:30 pm Randy's mom calls Bob's family
Sunday Sept 18 1977: car found at Tower Rd, Baraboo, WI
Thurs Sept 22 1977: tires found near Prairie du Sac.