- If you enjoy books with happy endings then you are better off reading some other book.
- —Lemony Snicket, The Bad Beginning
A Series of Unfortunate Events is a series of novels chronicling the lives of three orphaned siblings; said lives could be described as a series of unfortunate events.
The main canon is a tridecalogy (meaning "thirteen-part") set of novels published between 1999 and 2006. While actually written by Real World author Daniel Handler, the series is presented as the work of in-universe character Lemony Snicket.
There are other supplementary books, the most important being All the Wrong Questions (2009-2015), providing backstory for Snicket, and The Beatrice Letters (2006), which fleshes out some unexplained mysteries. There was also a movie that incorporated the first four stories, as well as an award-winning Netflix TV series.
The Baudelaire children are Violet, a tinkerer with a MacGyver-level ability to throw devices together; Klaus, a book nerd with a talent for research; and Sunny, a baby with powerful jaws and sharp teeth. After the mysterious, fire-related deaths of their parents, the Baudelaire orphans are sent to live with their (geographically) closest living relative, the plainly-evil-no-seriously-look-at-the-dude's-face Count Olaf.
Olaf is clearly after the orphans' inheritance money, and repeatedly tries to kill them. Because all adults in this universe are either blatantly evil or blithering idiots (though sometimes both), the Baudelaires' complaints about Olaf's murder attempts and use of the children as servants fall on deaf ears. Until Olaf tries too hard and makes himself look like a neglectful parent, anyway.
The Baudelaire children then get shuffled around from relative to relative, with Count Olaf following behind and leaving body after body in his wake. Along the way, the children come to learn more and more of their parents, and a strange secret society they had been a part of.
Lemony Snicket's writing voice in A Series of Unfortunate Events is famously dreary and pessimistic, sometimes even more negative than what is actually befalling the orphans at the time. This style can be difficult for less experienced fan authors to emulate, which can potentially throw Lemony Snicket himself OOC, if he's still the narrator in the Fic.
More adult authors will pair up Violet and Klaus, often dialing up Count Olaf's abuse to overly-dramatic and unfunny proportions to make them more desperate for affection. Olaf himself is sometimes made physically or sexually violent towards either Violet or Klaus, again being cast as far more realistically abusive than the tone of the series allows for.
Sunny, thank goodness, is almost completely free from all this, being a baby.