This book is about Duke selling gunpowder to the Trading Companies… signed by 'Umbra'.
The skeletons once began hoarding huge amount of gunpowder, known since as 'Black Powder Stashes'. The Trading Companies didn't like that, so Duke of the Bilge Rats saw a chance to make the seas safer while also making a lot of gold. It seems this was Duke's first step towards acquiring the connections and capital needed to launch the Black Market.
Hidden Spring Keep