The new book for the new audio... plus the old audio.
You could get the updated audio. I'd say that's the cleanest way to go, and I've recently found that the new audio is better. And yes, I paid for the new series myself, too.
If money is a consideration, and let's face it - it always is for me - you could listen to the old audio and apply that how you see working best to the new book. The new series is just expanded and edited, updated, but the central ideas and therapy strategies are the same. So you could do it this way.
Doing something, whatever you choose to do, still better than nothing, right? Yay!