If you're not sure about the idea of using sign language with your baby or not, I think this book will convince you of its utility.
I had made a half-hearted attempt at using sign with my daughter when she was very little but gave up because I didn't feel it was working. A few months later a friend lent me this book after telling me how excited she was about using sign language with her son. I suddenly realized that my daughter was already using at least one sign - to tell me she wanted to be picked up.
I read the book cover to cover, watched the video (unfortunately not available if you're in the UK), got my husband to buy in to the idea, and learned a few signs to get started. We wish we had started in earnest earlier! My daughter was quickly able to tell us lots of things that we would never have known or understood . For example, long before she could say 'plane' she could sign it. One day we were in the house and she suddenly signed 'plane'. I was initially confused, but then realized that she was telling me she could hear a plane. The sign language did not delay her speech but seemed to help it. She often learned the sign first, then used it together with the word, and now she only uses the signs for emphasis. The signs often helped us understand what she was trying to say as she was learning to speak - less frustration for her and for us!
I recommend the book as an intro to using sign with your baby, but found it much easier to learn signs from the video or from the Quicktime videos on the ASL webbrowser at the UMichigan website. We stuck to using 'real' signs as much as possible so that there was more chance of other people understanding my daughter if she used the signs with them. Many of them are really obvious anyway. My daughter made up her own sign for shower - then I looked it up and found that the sign she had come up with on her own was the real ASL sign for shower!