Book 3a of the Key Words with Peter and Jane reading scheme focuses on the activities that Peter and Jane enjoy, introducing 36 new words like 'me', 'tea', 'bed', and 'give'. After completing this book, the child advances to book 3b. The scheme is based on gradually introducing and repeating key words to help children identify them on sight, using the 'look and say' method. Examples of key words include 'the', 'one', 'two', and 'he'.