The Thirteen Gun Salute, the thirteenth book in Patrick O’Brian’s Aubrey-Maturin series, is not quite as stunning as The Letter of Marque, but its staggering cliffhanger of an ending will make up for quite a lot. You’ll want the next one on hand.
This book is divided into two parts, and events from the first part are of a type with other and more significant things that happen in the second (though the two sets are only connected by theme, and don’t necessarily have the same results). For whatever it’s worth, my mental picture of the structure is something like o O! (the exclamation mark is the cliffhanger). When I realized this, it helped me understand, in retrospect, the shift in plot that happens roughly a third of the way in.
The other non-spoilery thing of note is the remarkably sharp contrast between animal and human societies, for lack of a better word. I don’t think the book draws explicit conclusions or morals, but this is the first time I’ve noticed such a close juxtaposition, which makes me wonder if there is A Message.