I think it would be hard to really learn yoga by book OR by tape if you've never done it. You would get some of it, but a lot of yoga is about subtle alignment cues, and adapting poses to suit your particular body and abilities. Also, I find that MOST tapes go too fast!
Having said that, I would recommend a book called Yoga for Wimps
. This is a good introduction to the concepts behind yoga, and the poses presented are sort of pre-yoga poses with lots of adaptations.
A decent yoga video is Yoga for Weight Loss
put out by Gaiam. You can order directly from the company, or I bet you could find used copies on places like ebay, or on Amazon. I like this because it's not too fast, and they show lots of adaptations.
A great place to explore yoga online is the web site for Yoga Journal
. They have LOTS of articles and how-tos geared towards beginners.
I really recommend trying to find a class, though, at least while you are learning the basics. Keep in mind there are lots of different styles of yoga and yoga teachers, so talk to the teacher before class and find out what the style is and let the teacher know your limitations, if any. Don't be embarrassed about being overweight -- I started taking yoga classes when I weighed 240! And my teacher couldn't have been nicer or more helpful.