In reply to pasbury:
I had the Puffin Post from 1970-1975. The back cover of these used to show the front covers of a dozen forthcoming titles. I've just gone along memory lane (remember Flat Stanley? the Swallows and Amazons books, the John Christopher titles etc?) and looked at all my surviving copies, but I can't see anything that quite fits your bill.
Might it have been a compendium of Grimm's Fairy Tales? I bought a thick, grey Routledge paperback of this in 1984. or something on a Norse/Greek legends theme by Roger Lancelyn Green/C. Walter Hodges/Rosemary Sutcliff?
Or might it even have been one of the volumes of the Lord of the Rings? Have a look at images of Pauline Baynes illustrations to see if any of them jog your memory.