I disagree with your book's suggestion. Children don't learn spoken language by their parents focusing specifically on one word until that's mastered. Instead, learn a small group of simple signs and use them consistently. Just as with a child's first spoken words, the first sign can be individual.
The practical signs like those people have suggested:
are great, but I really suggest adding in a few observational signs which may have no "practical" importance:
Once when my daughter was one, before she had any spoken words, we were in the subway station. We had been near the waterfront, and she had been signing about the birds there. In the station, she suddenly started signing "bird" very emphatically. I laughed and said that there weren't any birds underground.
She kept doing it, and I started to think that maybe she was signing "milk" (since she was small, her versions of the signs were kind of similar — they're both open/close motions and hand position is difficult), but she shook her head to that and signed more, and pointed at up at a light in the rafters.
I laughed and said "That's a light, not a bird" — but then I realized that there were actually two small sparrows perched there. So I told her that she was right and that I should not have doubted.
Here, no needs were met or particular purpose served, but yet we had a nice shared moment of communication. I don't think this would have happened (at least not so soon) if we'd just been focused on more or milk.