Gestalt Therapy, Frederick Perls, Ralph Hefferline and Paul Goodman  

Gestalt Therapy Verbatim, Frederick Perls  

Ego, Hunger and Aggression, Frederick Perls  

Effective Psychotherapy, Helmuth Kaiser  

The Psychiatric Interview, Harry Stack Sullivan  

A Brief Introduction To The Genius Of Nietzsche, Richard Chessick (This is out of print but available from the usual resources)

The Antichrist and The Birth Of Tragedy, Fredrich Nietzsche  

The Freud Reader, Peter Gay  

If You Meet The Buddha On The Road, Kill Him. Sheldon Kopp  

This Side Of Tragedy, Sheldon Kopp  

The Hanged Man,  Sheldon Kopp  

The full 24 volume edition of The Standard Edition of the Complete Psychological Works of Sigmund Freud is available from Karnac Books for approximately $550.00.  This is the full 24 volume set in paperback editions and is an excellent value.  

The Complete Works of Harry Stack Sullivan is also a highly valuable resource.  It is long out of print but is available used from many sources.  It is a two volume set and can usualy be found for ~$50.00.   In addition these texts are also helpful readings in conceptualizing the Gestalt Therapy framework.

Nature Heals
Paul Goodman

Here, Now, Next: Paul Goodman and the Origins of Gestalt Therapy
Taylor Stoehr

Community and Confluence: Undoing the Clinch of Oppression
Philip Lichtenberg

Encountering Bigotry: Befriending Projecting Persons in Everyday Life
Lichtenberg, van Beusekom and Gibbons