RE: PIC32 assembler
the PIC32 like most 32-bit microcontrollers is ideally designed for programming in a high level language like C or C++, but if you are absolutely thirsty for performance I can understand your desire to get to know the MIPS core assembly language.
A good starting point could be: MIPS Assembly Language Programming (Paperback) by Robert Britton
( ISBN-10: 0131420445 or ISBN-13: 978-0131420441)
You can find it on Amazon new or used as well as I am sure in many other bookstores ...
Hope this helps