It's an alternate history/American Civil War/naval fantasy!
Lieutenant John Neavis, kept far too long at desk work, receives his new assignment with delight: aboard the USS Nicodemus. Though it does start to look odd when he gets the cannons to be delivered and hears that they, and the cannon balls, were all ordered in brass. And virgin brass at that, never before cast into any other form. And when he heads, far to the north, to deliver them, he is ordered to give up all the iron on his person, down to the boots that have nails in them.
And Miss Columbia Abrams, after a dinner where her father's guests include Cornelius Vanderbilt, finds herself on board the same ship for certain purposes. Her dedication to the causes of Abolition and Union make her resolve to do her duty as befits a daughter of the Commonwealth of Massachusetts and a graduate of Hedley Female Academy.
The captain was never a sea-going man before, and they find more and more uncanny and unpleasant things about him. On the other hand, sailing after Confederate ships, they find one ship in particular, and its captain with it, are even more uncanny and unpleasant.
An epistolary novel. Mostly the accounts are from Lt. Neavis and Miss Abrams, but other documents are interpolated to fill in the appropriate information.