Basic English Grammar

I am using Azar's Basic English Grammar in my ESL instruction, and find it to be a solid text with plenty of useful examples and student drills and assignments. The subject matter is useful and practical, designed for the mature 'teen or adult. Azar also includes many "real-world" usages that give ESL students -- and even native speakers, in many cases -- lots of trouble. Rules are often presented in an arbitrary fashion, leaving clarification up to the instructor. I don't mind this, though. Azar provides the "what" and I provide the "how" and "why."