The language in books is not the same as the language of conversation. In books, sentences are more complex and the vocabulary is rarer. Adults can help young children understand book language by choosing books on topics that are already familiar. If children have experienced and talked about something, then a book that is related to the experience is a good choice to read aloud. For babies this means reading books about pacifiers, diapering and mommies. The wider a child's experiences, the more topics they are capable of learning about in books. New language gets mapped onto concepts that the child has already mastered.
Ask your child to compare his or her experiences to the experiences described in the books.