In an effort to track down the cover of my first LotR, I came across
this site for collector's of Tolkien's works, which has picture of covers from old and rare editions.
And I was overjoyed to find the edition that I first read, that is now, alas, long lost. It was the first English paperback edition in one volume, published by Allen & Unwin 1968, and looked like this:
There are some other editions of the books there that I recognise too, either from friends copies or from descriptions here. Although it's not comprehensive, the site is well worth a quick visit.