Full sets (I only have Planets, Man and Space, Energy, Mathematics, but there's apparently 26 in total) do occasionally come up on Ebay for no money plus infinity shipping.

Some of the stuff in "Planets" was sufficiently advanced -- and sufficiently carelessly-lost between the 1960s and the 1990s -- that a friend of mine actually spent a proportion of his PhD rediscovering some of it, and was understandably galled when I let on that some of his conclusions were already contained in the astronomy book I grew up with.

Peter