The Five Find-Outers is series of mystery novels by Enid Blyton, consisting of 15 novels and running from 1943-1961. It is one of her more popular series, being for an older audience than The Secret Seven, but very much of the same cut. It is targeted at older children and young teens, perhaps, and very roughly, 8-15 years of age.
The Five Find-Outers are formed when Larry, his little sister Daisy, and their neighbors Pip and Bets start to investigate a burnt-down cottage, and meet a stuck-up, rather fat rich kid -- Frederick Algernon "Fatty" Trotteville -- who has a nice dog, Buster, and likes to investigate mysteries as well. After a rocky start, they team up, and soon they are solving mysteries left and right.
Unlike The Famous Five, the The Five Find-Outers have fairly normal lives in a rather small village, and there are no smugglers, secret tunnels, mad scientists, spies, other other such nonsense. Granted, there are more jewel thieves than is statistically credible, but the children spend most of their detective work on actual detecting -- stake-outs, eliminating suspects, collecting clues, and other proper, no-nonsense things that any kid could do (hence they appeal). They do, however, have one specialty: Fatty is a master of disguise, and pretty much never gets found out when he's dressed up.
The series has another gimmick, although it's one found frequently in Blyton's books: the local cop is a buffoon, and the children mock him constantly. Sometimes they simply test out new costumes on him, sometimes they prank him, sometimes they break into the his house and steal evidence in ongoing investigations... you know, harmless kids' stuff. The aptly-named Mr. Goon is actually quite awful (you can tell because he kicks at Buster whenever the poor little dog tries to nip him), but it is also the case that the children can be quite awful.
It should be noted that, because this is the 1950s, things sometimes get a bit politically incorrect; gypsies are thieves and scoundrels, poor people are stupid and unhygienic, and the five are happy to use insults that would get them labeled as the nasty bullies in a modern novel.
Overall, good, fun novels, but not the best writing, very definitely formulaic at times, and not attuned to modern sensibilities. Pretty much par for the course as far as mid-century children's adventure stories go.