Since the terms of the Plusnet line rental contract don't stipulate receipt of a Phone Book, there doesn't seem much else that can be done.
Interesting that Plusnet are staying so quiet about this.
Given that their wholesale supplier specifically states that end users are entitled to a FREE BT phonebook I'd be very surprised if Plusnet will refuse individual requests.
Also this thread will provide information to those thinking of switching from BT. It appears that if they move to Plusnet they may lose some benefits they currently enjoy (or not).
Plusnet unlimited FTTC