I've gotten addicted to those Stratemeyer Syndicate novels (and their ilk) meant for YAs. I started to read them because they were some of the only things written in the early 1920s. I've got a writing project set there and thought it would be good for background, especially WW1 stuff (which is obviously toned down a lot for its intended audience, but enough filters through that it's still interesting).
But then I just got hooked.
Very formulaic, even the titles.
Air Service Boys
http://manybooks.net/authors/beachcha.html
Brighton Boys
http://manybooks.net/authors/driscollj.html
There's actually like a dozen different "_____ Boys"
Ruth Fielding (was a nurse for a while)
http://manybooks.net/authors/emersona.html
Penny Parker (this is when I realized I was hooked, set in the 30s and 40s, more like Nancy Drew, the author used to write those)
http://manybooks.net/authors/wirtm.html