Let's start with the same as Erik stated: most manufacturers don't want to publish detailed information on parallel programming, so unless you run an well established business in making programmers, it's unlikely you get it. But some chips do have published parallel programming specification usually directly in the datasheet, try looking at some of Atmel's offering.

Physically, parallel programming is accomplished in a very straighforward manner: usually triggered by a certain combination of logic levels on certain pins, often involving setting one pin at a "high" voltage (above the high logic level, usually to around 12V), multiplexers on pins detach the microcontroller's circuits and attach the memory's interface, so then the chip looks just as if it would be only the memory. You know how FLASH memories are programmed, don't you?

JW