I think it's very dangerous to learn
jo and ken from books or video tapes.
You should find a good teacher who
knows the relationship between aiki
jo(ken too) and aikido techniques.
Or you'll learn three different things.
It's really dangerous... -.-
While it wouold be difficult to teach yourself from a tape without having an experienced teacher, I am a very big believer in using them for your training. For years after I left Saotome Sensei's dojo in Washington, DC I had no one to train with in weapons as the dojo where I was enrolled did not do any weapons training. I purchased every video I could find (which in those days wasn't that many) and worked on the techniques. I don't think I did myself or any body else any harm by doing this. I have a hard time understanding what is meant by "dangerous" in this context. The worst thing that seems to happen can be that you don't understand what you are looking at and practice some techniques incorrectly.
I still rely on videos to give me new material to work on. The student who has been exposed to a wide range of possibilities is going to learn faster and I encourage my students to get as much exposure as they can. Alas, many of them see fit to wait until I get around to showing them rather than go after it on their own. It's the really hungry ones who are going to be the best one day.