Blaz, Deborah. Differentiated Instruction: A Guide for Foreign Language Teachers. Eye on Education, 2006. This book has a wealth of strategies for differentiating instruction in the world languages classroom, as well as a thorough explanation of different methods of differentiation.
Boyles, Nancy. Constructing Meaning through Kid-Friendly Comprehension Strategy Instruction. Maupin House, 2004. If you choose to do any reading comprehension activities with your students, this resource is invaluable.
Curtain, Helena and Dahlberg, Carol Ann. Languages and Children, Making the Match: New Languages for Young Learners, Grades K-8. Allyn and Bacon, Inc., 2003. One of the seminal works for K-8 language teachers, in its third edition. I always refer to this and the Omaggio text when I have questions of a theoretic or strategic nature. In particular I consulted this work for Krashen's discussion of Comprehensible Input.
Marzano, Robert J., Pickering, Debra J., and Pollock, Jane E. Classroom Instruction that Works: Research-Based Strategies for Increasing Student Achievement. ASCD, 2001. This is one of the educator's bibles for quality instructional strategies.
Omaggio Hadley, Alice. Teaching Language in Context. Cengage Learning, 2000. The other seminal work for language instructors!