Being a Boomer, I'm now on a nostalgia kick for those old books, and as many of my peers are now down-sizing, a lot of the books are available online and in used book stores. I have quite a nice collection of the ones that came out from about 1954 through maybe 1962 or 63 ... I will stretch it as far as 1966, though I was well past the club age by then, if the book seems in the same spirit as those 1950s works.
My most recent acquisitions:
I remember reading Secret of the Old Post-Box but not Surprise Island when I was a kid. I probably did, I just don't remember it. I have also recently got a collection of 7 of the Cherry Ames nursing books. I'm going to make a slipcase for those, and I think I'll make slipcases for the Weekly Reader Book Club books as well.
Many of those old WRBC books still hold up well; some, as in The Secret of the Old Post-box, disappoint. It's almost all Tell, very little Show. Surprise Island is interesting because the main character, the girl, is a pretty obnoxious protagonist! But I think it's valuable because she does progress during the story to where she becomes able at least to see that she HAS been pretty thoughtless and overbearing, and wants to change.
I love the pen and ink drawings in these old books, too. Of course, I would love it if my granddaughter would enjoy reading these things at least once, to give her an idea of what her old granny loved to read when she was that age, and to reveal what life was like before the Computer Age. But if she doesn't care for them, at least they've been preserved for a few more years and maybe someone else will get them, down the road.