Direwolf20 Code Not Working?

I am using the code and setup from direwolf20's video: http://www.youtube.com/watch?v=_OSmP5pUK9Y
On him making a mystcraft portal with montitor as a sort of selection of where the destination of the portal will go.
I have done everything the same as he did apart from change the location of the computer to the right instead of the left and changed that in the code and the monitor seems to be working but on the computer it is not sending back the info of the books I have put into it. It says "books:17: attempt to call nil"
Anyone know how I can fix this?
Thanks
Krazy

Someone else has recently had the same problem with (what I assume is) the same code.

Ok. Thankyou. Im not very good at computercraft but I'd give it a go so if you find a solution could you post it here please? Its just when I read that post you linked to it confuses me even more :wacko:/>
Thanks
Krazy