"Tell Me Why" books were meant to offer up answers to kids' inevitable and constant questions about the world in those pre-Internet days.
This Metafilter question says that there was a 1960s series and then they were revised in the 1990s. The poster doesn't care for the modern version, as with so many revisions, they kinda took the charm out of them.
If you didn't have these books, what works did you turn to as a kid for reference? Did your folks have a full set of encyclopedias, or other general reference books, or did you practically live at the library?