A few of my favorite soul-hacking writers are:

William James – turn-of-the-century psychologist, philosopher, and one of the first Westerners to say, as you put it, “truth is irrelevant unless it changes who you are in some sense”. His “Varieties of Religious Experience” helped shape the direction of my religious thought and has an honored place on my bookshelf.

Robert Anton Wilson – discordian, drugged-out-hippie, and proponent of what might be called “gonzo soul hacking”. He taught me the value of believing six contradictory things before breakfast.

Alan Watts – but I see that I need say no more on that score :-) I’m currently working my way through “The Book”.

Thomas Moore (Care of the Soul) and James Hillman (The Soul’s Code) – contemporary depth psychologists and advocates of thoughtful soul work.

Eric Raymond, believe it or not – my favorite of his essays is not about software at all, but about soul-hacking from a (software) hacker’s perspective: http://catb.org/esr/writings/dancing.html