A Series of Unfortunate Events: The Hostile Hospital, Lenomy Snicket (aka Daniel Handler), 2001
To be honest, I don't remember much of this book other than this is the one where the kids find a page in a file that suggests that there was one survivor of the Baudelaire fire, but it doesn't say who that survivor might be. Instead, there is just a reference to another page of another file that is never seen.
I guess V.F.D. was getting old as a macguffin, so he switched to the search for a parent, which at least gives the three kids some hope, unlike the search to discover the meaning of the letters V.F.D.
Lots of anagrams and cultural allusions for the adults. I think I remember enjoying this one because I thought that they were getting somewhere ... sooner or later. And I was somewhat surprised because the existence of a parent would be a very fortunate event, which is out of place in this series.