Stratemeyer Syndicate

The Stratemeyer Syndicate created many many formulaic juvenile book series during much of the 20th century. Each series was written by many ghostwriters sharing a common pseudonym.

Edward Stratemeyer, the head of the syndicate, took a rather direct role in the creation of many of his books, which may be rather surprising considering how basic they are. He invented the primary characters of his stories by listing little more than a name and a basic description, and letting the ghostwriters fill in the personality blanks to flesh them out. He created rigid plots, but left enough blanks in the details to be filled out by a creative writer. Stratemeyer's books were super formulaic, and the man himself tightly controlled the formula.

Virtually all of the book series were about teens going on adventures or solving mysteries, with slight variations on the concept. As such, the books contained very similar themes and portrayals. Characters had platonic love lives, if any at all (rather humorously, this led to the Alternate Character Interpretation that the Hardy Boys were gay, due to their lack of interest in their nominal girlfriends, preference for male friends, and one brother's close friendship with a boy who disliked girls). Suspense was used to heighten tension, but violence was limited—characters could get knocked out or tied up, but nothing worse than that. Language was tame, and even expressions such as "oh gosh" and "oh golly" were dropped after some readers complained that they were merely euphemisms for "oh god".

Stratemeyer was a marketing genius if nothing else. He noticed the changing times and applied them to his new book series. When the adventures of undersea diver Dave Fearless were losing popularity, Stratemeyer created the Hardy Boys to take their place, with a greater emphasis on dialog and character. When the women's lib movement started, Nancy Drew came into existence, and became hugely popular. The addition of Jewish and Italian characters to The Hardy Boys was a response to America's growing tolerance for diversity at the time. Notably, the characters' only real personality traits in Stratemeyer's original description was that they happened to be Jewish and Italian; the ghostwriter had to give them actual personalities.

The Stratemeyer Syndicate's series include

The Rover Boys
The Bobbsey Twins
The Hardy Boys
Nancy Drew
Tom Swift
Dave Fearless
The Dana Girls
Trixie Belden

The Stratemeyer Syndicate provides examples of the following tropes:

Adults Are Useless: Done intentionally by the most prolific ghostwriter, Leslie McFarlane, who believed that kids should be exposed to corrupt and incompetent authority figures in fiction, so that readers didn't become too reliant on them in real life.
Bound and Gagged: Often in lieu of "real" violence.
Crossover: Didn't start happening until after Edward no longer ran the company, but the Hardy Boys and Nancy Drew have met each other many times since.
Extruded Book Product: Nancy Drew and Hardy Boys have often been called "anti-literature" for precisely this reason.
Kid Detective: What most of the books were about.
Kid Hero: Again, what most of the books were about.
Sibling Team
Snooping Little Kid: How the kid heroes get into danger. In Hardy Boys in particular, the boys would do their own parallel investigation separate from their dad, a detective himself.
Strictly Formula