Just in case you didn't know (I refer you to this post, this post and this blog), I'm a big fan of The Three Investigators series of books. I started reading them in 1978 or so, have a full set of the first 30 (all the format B editions that were published, which I recently completed with "The Secret Of Skeleton Island") and still love them.
Once I'd completed my format B set (which took several years, lots of 2nd hand bookshops and ebay), I realised that I'd actually enjoyed the hunt so decided to try and get as many format A editions as I could (I have about half a dozen or so at the moment).
I only have one hardback, the "Skeleton Island" one that started everything off and it never occurred to me to try and collect them - if you check ebay, they're usually very expensive. But then, earlier this week, I found a Collins hardback first edition (from 1971) of "The Secret Of The Crooked Cat" on ebay and they only wanted £6 for it. Thinking I couldn't lose, I bought it and it arrived today and it's fantastic. The cover only has a couple of dinks in it, the pages are white and clean, there are no markings and the line drawings (by Roger Hall) are the clearest I've ever seen them.
I now have the story - one of my favourites of the series - in three editions.
The Collins Hardback First Edition was first printed in 1971 (the one I have) and last reprinted in 1973. The cover art and internal illustrations were by Roger Hall.
The Armada Paperback (Format A) was first printed in 1973 (my version) and last reprinted in 1980. The cover art is by Peter Archer, with the same interior illustrations as the hardback, by Roger Hall.
The Armada Paperback (Format B) - the one that my collection is made up of - was first printed in 1982 (my version) and last reprinted in 1983. Cover art is by Peter Archer again, with Roger Hall's interior illustrations reprinted from the hardback.