D.K.
I can't give you any names of books on this subject, but I can tell you about my son. He is bilingual. My husband is Japanese, and we live in Japan. Most 9month olds can't really speak much in a first language, let alone a second. From the day my son was born, my husband and I have only spoken our native languages with him. My husband only speaks to my son in Japanese, and I only use English. I am a stay at home mom, so my son spends the majority of his time with me. My son said his first word when he was 15 months old. His first word was in English, his second word is the same in English and Japanese, and after that words were added randomly. Learning a language comes from being exposed to that language. Listening comes first. Talk to the child. Narate every small detail. This is what I did with my son. I would narate everything I did. Example: "Let's eat our breakfast now. Today, we will eat a banana, some rice cereal, and some carrots. Look, I am mixing your formula into the rice cereal. Let's put it in your "winnie the pooh" bowl. Mommy's going to peel the banana now. Are there any bad spots on the banana? I found one. Let's cut that out. Now, I am going to slice the banana." And so on... I talked to him non stop most of the day. I also read to him a lot starting from when he was 6 months old. I read 3-5 books to him before his morning nap, afternoon nap, and bedtime. He is four now. He understands English and Japanese fluently. He prefers to speak English, because he spends the majority of his time with me. Recently, I have noticed that he is speaking more Japanese with his dad and other Japanese speakers. He can tell you if someone only speaks English, only speaks Japanese, or can understand both languages. He can translate what his dad says to him in Japanese into English for Mommy to understand. He is learning different vocabulary in each language. Sometimes it overlaps. Sometimes it doesn't. He loves to tell me what his library books are about. His library books are all in Japanese. He is learning to read in English now. He can sound out words, and knows about 100 sight words. He can read through some easy books with a little help. He can't read at all in Japanese yet. That will come when he enters elementary school. For now we are focusing on one language for reading and writing. The one, he won't get once he starts going to school. My friends who have bilingual families with children older than mine have all told me that their kids were between 5 and 7 years old when they started rattling off in both languages equally. The boys tended to take a little longer than the girls. The best advice that I can give you is to talk until you are tired of hearing your own voice. Language learning doesn't happen over night, over a month... It takes time and exposure. Hope that helps.