The publication date on this book is unfortunately still in the future. Young hand-fed African grey parrots may start to mumble human sounds at a few weeks of age. Some greys start talking at a young age while it is not unusual for others to start talking after they are over a year old.
Parrots learn to talk much like small children.
When they hear certain words over and over, they start practicing the sounds. At first the sounds are simply incoherent mumbling but the proper inflections are often there. This is the origin of much 'baby-talk' in humans. Just as small children need an adult to listen carefully to pick up what the child is trying to say, young greys also need a human to reinforce the words they are trying to learn and teach them to say the words correctly. Pay close attention to the mumbling and you will begin to hear the words as they develop.