Kristin Ciullo You can read the book without having read A Series of Unfortunate Events but there are cameos and easter eggs that readers of ASoUE will get. Still entirely enjoyable as a stand-alone series.
eden I have only read the first three books of the Series of Unfortunate Events, and the first two books of the Who Could That Be At This Hour series, but I don't think the two are connected; same author, different characters. So yes, it would be okay to read this series before finishing the other (like me!!).