Hi,
Doing my own thread necromancy here since I finally found an answer to my question!
(My old log-in wasn't working for some reason, so had to create a new similar one.)
I recently found the website,
http://www.myfonts.com/WhatTheFont/
Scanned in one of the 1973 Ballantine book covers, and that site quickly identified a number of possible matches.
The ones I like the best are:
Tribunus SG Roman
Tribunus SG Bold
Also, following in order of my preference:
Cadmus
Hadrian
Linotype Trajanus Roman
Thought I'd tidy up this thread in case anyone else is searching later.
Cheers,
Maureen