Yeah, I would never want to use just the book. The lecture is more detailed, conversational, animated, and interactive, so it engages the kids more and makes everything easier to understand. There are prompts during the lecture for the kids to fill something in to show they are getting it, etc.
The lesson in the book just summarizes what's in the lecture.
Then when the child fills in their answers on the computer, they can have an animated buddy who will sometimes give hints and will always give encouragement for correct answers. If they get it wrong, they have a chance to try again in case it was a simple error, and if they still get it wrong, they have the option to "view solution," which shows and tells step by step what they should have done to get it right (mandatory here if you get an answer wrong). It keeps a gradebook of all the lessons and quizzes. It has some cute cartoons to further engage the student. To me, TT without the CD's just wouldn't be TT and I wouldn't bother using it.