People who meet criteria for this disorder experience significant difficulties with anxiety but do not meet the full criteria for any of the specified anxiety disorders such as Generalized Anxiety Disorder, Panic Disorder (and Agoraphobia), Specific Phobia, and Social Anxiety Disorder. They typically have symptoms that meet partial criteria for one or more of the anxiety disorders. For example, someone with this disorder may experience excessive worry, difficulties concentrating, occasional panic-like symptoms, and avoidance of certain situations, but they may not meet full criteria for Generalized Anxiety Disorder or Panic Disorder.
Many of the therapy protocols and self-help books are written for people who meet criteria for one of the specified anxiety disorders. It can be a bit of a challenge to adapt these protocols and books when someone meets criteria for unspecified anxiety disorder. This also is an issue when a person meets criteria for two or more specified anxiety disorders.
Fortunately, there are a lot of common features of the psychotherapies for all of the anxiety disorders. People with any anxiety disorder all benefit from using the principles of cognitive reappraisal, cognitive defusion, and exposure therapy and by learning mindfulness and relaxation skills.
Good self-help books for people who meet criteria for unspecified anxiety disorder (or any anxiety disorder, for that matter) are:
Forsyth, J. P., & Eifert, G. H. (2016). The mindfulness and acceptance workbook for anxiety: A guide to breaking free from anxiety, phobias, and worry using Acceptance & Commitment Therapy. New Harbinger. [Amazon] [New Harbinger - Free Accessories]
Tompkins, M. A. (2013). Anxiety and avoidance: A universal treatment for anxiety, panic, and fear. New Harbinger. [Amazon]