First of all, there is the "UNITY book":
K. Mani Chandy and Jayadev Misra. Parallel Program Design: A Foundation. Addison-Wesley, 1988.
Then, Beverly's paper on the substitution axiom:
Beverly A. Sanders. Eliminating the substitution axiom from UNITY logic. Formal Aspects of Computing, 3(2):189-205, April-June 1991.
Jay Misra, and other people in Austin, Texas, wrote the "Notes on UNITY", a series of 34 papers about various aspects of UNITY.
More recently, Jay Misra wrote the "New UNITY" papers:
Jayadev Misra. A logic for concurrent programming: Safety. Journal of Computer and Software Engineering, 3(2):239-272, 1995.and
Jayadev Misra. A logic for concurrent programming: Progress. Journal of Computer and Software Engineering, 3(2):273-300, 1995.Stuff about New UNITY is also available from Austin as technical reports.
You can check the bibliography of my PhD thesis for a number of other references.