I ordered the Discipline of Spiritual Discernment on Amazon a couple of weeks ago, and the book arrived today. It's a little thinner than I expected, and I like it that way, being painfully slow reader.
On a side note, I actually got it delivered to my work, but I didn't expect my boss to bring to my desk! It was weird receiving from the hands of the head of Koorong a book bought on Amazon. Dang...
You can find some reviews on this book by googling, and here are some I read.
I'm sure Koorong will have them pretty soon, so you can buy it there.