You can get the instruction set from microchip website.
Search for the reference manual in microchip website. This will fetch you the full instruction set and its the reference manual for the pic microcontrollers.
And Coming to Hi-Tech C. You should know ANSI C well. And something about PIC Microcontroller. Read the manual provoided in the Hi-Tech C folder. I think this is suffucient to learn Hi-Tech C.
ask for the PICC quick start guide
HI-TECH sayes "We do have some quick start guides that might help. These are available for our PRO compilers, but they would help with the STD or Lite mode compilers as well."
please create a movie for HI-TECH PICC from begain installing using HI-TIDE with "Hello world" simple programme. it should help us who want to migrate to "c".