I suggest that every teacher who teaches ESL should have the book Learner English. It is the best ESL manual I have ever used:
I also suggest every teacher should have an understanding of Total Physical Response (TPR), so I would include information on this.
A good picture dictionary for students and teachers.
Paste on labels for items in the classroom--pencil sharpener, desk, window, door, etc.
An example of a general ed lesson plan with ESL modifications/accommodations added, to show how the regular ed curriculum can be modified/accommodated to reach the ESL student.