David H. Barlow 
Anxiety and Its Disorders [PDF ebook] 
The Nature and Treatment of Anxiety and Panic

This landmark work is indispensable for anyone studying anxiety or seeking to deliver effective psychological and pharmacological treatments. David H. Barlow comprehensively examines the phenomena of anxiety and panic, their origins, and the roles that each plays in normal and pathological functioning. Chapters coauthored by Barlow with other leading experts then outline what is known about the classification, presentation, etiology, assessment, and treatment of each of the DSM-IV anxiety disorders. A definitive resource for researchers and clinicians, this is also an ideal text for graduate-level courses.