I had forgotten about that excellent series of books. It felt as though I should have remembered something like that. After a fruitless topic search, I looked into period publishers which led me to the article. Still, it didn't ring a bell; until, I saw pictures of covers I recognized. The change in back cover design cinched it.
I didn't own these books which is probably why I didn't remember them at first. While in elementary (primary) school, there was a
library period during which students could study; or, roam. It was there that I encountered the series and these two covers (Gila monster and microscope) came back to me:
Then I remembered the change in back cover (with series index) design between different volumes and thinking it was quite an improvement. The article says the change occurred in 1962; but, I think I
née 55 didn't notice the series until 1965 when a mandatory Spanish language class in 5th grade introduced me to the Gila monster as an example of letter pronunciation and piqued my curiosity in the library. Shame I hadn't remembered all that without cues so as to make it my first suggestion. Oh well, better late than never.