I'm an Occupational Therapist, not a Speech Pathologist, but I know that babies can sign before they can speak. It doesn't matter what signs you use as long as they are consistent and easy. I am going to just use regular sign language with April, but that is because I can sign already. In reality, you only ever get about 5, maybe 10 signs before most children can talk anyway. Some of the best words to make up signs for are more, eat, drink, finished. Other ones are cute but not so functional like bath, sleep, book.

Don't be put off buying the book, I'm just not sure you will get enough use out of it. Why not try your local library for either the actual book, or any book on sign language. You can then just start with the basics and see how you get on.

Hope that helps