Book 4 of The Second Journey of Agatha Heterodyne. Do not try this one alone without the earlier books -- though they do give the account of what has gone before and the character list
For it starts out with going to a party, but -- this being Agatha -- does not end there. The offer to become her biographer is simple enough but there is action and adventure aplenty in Paris. There are revenants, messages diverted, an unexpected guest whom we recognize from earlier adventures, the backstory of the original Storm King, Martellus behaving with unexpected nobility, Tarvek dressed as a jester, and more.
The political intrigues all build in this one from many characters we have seen before.