That's a really good question. I think its "The Large Encyclopedia of the Tarot". My guess is that its an earlier edition of his now 3-volume set. Here's a little more info including year of publication and publisher.
Kaplan, S. R. (1978 ), La Grande Encyclopédie du Tarot, trad. de l'anglais, Paris, Tchou. ISBN 2-7107-0142-1
Given that you give a 1978 date, I suspect that it is only volume one The Encyclopedia of Tarot, first printing 1978, with just under 400 pages each a little shorter than standard A4 size.