I haven’t read the first in the series, but this can definitely be read as a stand-alone novel.
It doesn’t seem as if Mary does any real detecting at all. Solutions appear to her suddenly or fall at her feet.
I think it could have done with a little more direction and more detecting for that matter. Everything revolved around the who’s who of society and Mary rubbing shoulders with them. Even the potential love interest was shallow and flighty.
A nobody, as per society rules, with a Vanderbilt? Was it ever really going to happen? As soon as you take away the money things change. Love doesn’t seem quite so important when your lap of luxury is being threatened.
Of course without Vanderbilt Mary wouldn’t have had a direct route to the upper echelon, which is probably why her love interest isn’t just someone from her own walk of life.
Overall it was disjointed and suffered a bit from threads going off in different directions and not coming together very well.
This could be stronger with a little more focus on crime, perhaps a little less fancy-footing and social waffling.