Are you aware that the manual is available on-line?
For sure I know. But for sale is not an electronic edition, but the fully original Polish print. You will rather not get such a printed edition online.
Are there collectors of old phone manuals out there?
That's the purpose of the ad, locating such collectors. May be there are, who knows.
I would think old phones are definitely easier for intelligence services to monitor since it's only unencoded traffic
There's no traffic and no Internet on such phone. It's a cellular only. SIM-card could be tracked only by the mobile network operator company, and still not precisely - location would be determined somewhere between the closest station antenna which gives and receives signal from this phone, not the precise place. Nor Microsoft, Facebook and other companies would have access to determine where this number is currently located.