The Word of God answer (from a recent Reddit AMA) is that it's pronounced 'shanner-ruh' (ʃănărŭ) with the harder accent on the last syllable.
Questioner : I know, now, that the correct pronunciation is "SHAN-ar-ah". How much does it irritate you when people refer to it as
Terry Brooks : Hi Brent. How sneaky of you. No, I don't care much how people pronounce my names, so long as they can manage it.
He also goes into a little more detail in a recent Q+A
Terry Brooks replies: Here’s the thing, Bryan. I have deliberately avoided doing exactly what you are asking. Not that I haven’t been
asked to do it over and over and not that I haven’t thought about it
from time to time. But I adopted a hands-off policy early on in my
writing career when it came to Shannara. I didn’t what a definitive
vocabulary to facilitate pronunciation. I wanted readers to adopt
their own way of saying names, which I think makes a book more
personal to each reader. That’s why you never get a comprehensive
description of characters and creatures. I want you to imagine them. I
want you to be part of the process. If I put out a guide, it removes
that element of the reading of the books.
Admittedly, in retrospect, I probably should have done something about “Shannara”, which no one has pronounced the way I do since the
very beginning. I just thought everyone would say it like I do –
SHAN-NA-RA. Isn’t it obvious?
Guess not. My mistake, but I still think I will leave the creation of a comprehensive vocabulary for another time and place.
You can also hear the author's own pronunciation here: