Diane Larsen-Freeman & Michael H. Long 
An Introduction to Second Language Acquisition Research [PDF ebook] 

Understanding how people learn and fail to learn second and foreign languages is increasingly recognised as a critical social and psycholinguistic issue. Second languages are vitally important to diverse groups of people, ranging from refugees to college students facing foreign language requirements. This book provides a synthesis of empirical findings on second and foreign language learning by children and adults, emphasising the design and execution of appropriate research.