Leslie McDevitt's Control Unleashed website. She is the guru of dealing with reactivity in dogs and in building focus.
"Aggression: Before the Game Begins" by Pamela Dennison, is a good summary of the steps that can be taken to help a fearful dog not become an aggressive dog.
Grisha Stewart, MA, CPDT-KA is an author, seminar presenter, and dog trainer who specializes in dog reactivity and promotes force-free methods and functional rewards.
Emma Parson's website, author of the book "Click to Calm", which features some great articles to the thematic of fear and reactivity.
Julie Robitaille and Emma Parson together developed the TACT Program which stands for Touch Associated Clicker Training. TACT combines clicker training and massage therapy and develops confidence and coping skills while changing the emotional response in fearful dogs.
How To Treat Separation Anxiety in Dogs by Malena DeMartini Price. This book is highly recommended for separation anxiety by excellent dog trainers.