From Amazon for about $24 new. It received 5 stars from 12 customers.
Growing up with two Languages by Andersson & Andersson
From Amazon for about $30 new. It averaged 3 1/2 stars from 12 customers.
The Multilingual Mind by Tracey Tokuhama-Espinosa
Raising Multilingual Children: Foreign Language Acquisition and Children by Tracey Tokuhama-Espinosa
Parents’ and Teachers’ Guide to Bilingualism by Colin Baker.
See also: Raising Multilingual Children