The Happy Hollisters is a series of beloved vintage mystery/detective stories.
The five Hollister children, the protagonists and amateur sleuths in these stories, were actually modeled on the author's own children and he often based these books on events that had happened in his own family.
Apart from the wholesome fun these stories offer, they are also educational - which was intended by the author, Andrew Svenson (pseudonym, Jerry West). He always wove enlightening details into them and it was this combination of thrilling background information, of mystery and intrigue that made the series very appealing to young readers:
The Happy Hollisters ...
... and the Ice Carnival Mystery (a trip up North for the five Hollister children, to the Winter Carnival in Quebec).
... and the Punch and Judy Mystery (a visit "to the beautiful land of Italy").
... and the Mystery of the Little Mermaid (exciting happenings around the famous statue that overlooks the harbour in Copenhagen, Denmark).
... and the Castle Rock Mystery (it all starts when "a package of instruments from a weather balloon is parachuted onto their front lawn"...).
... and the Mystery in Skyscraper City (a tour of New York... and an old book of early New York tunnels...).
and many more!
Some select volumes have been reissued in paperback and eBook formats. Most of them, though, can only be found in the original hardcover versions (which I personally much prefer).