The Dance of Battle is an old book on war strategy.
It's described as a book with black leather binding so age-worn it is butter soft.
This book belongs to Cassian, and Nesta notices that he must have read it about a thousand times due to his scent and the age-worn look of the book.
When Nesta Archeron is training with Cassian and staying in the House of Wind, she used to go down the stairs of the house constantly to try to reach the bottom. One day after she comes back to her room after being on the stairs, she finds the book on her nightstand. At first, she thinks the sentient House has left it for her as it was common that the house left books for her to enjoy. She tells the House the book isn't the usual sort of romance it would recommend to her, to which she thanks it but tells her to take it back. The House doesn't act, and Nesta notices the familiar scent of Cassian drifts from the book's pages. She figures out it wasn't the House that left the book for her but Cassian.
Nesta is intrigued as to why Cassian left the book for her and deemed her worthy of its contents. She begins to read the book, not pausing her reading to even eat, devouring her stew with one hand while she holds the book with the other. It's midnight when she finally takes a break, astonished at how much the art of warfare was like the social manipulation her mother had taught her: picking battlegrounds, finding allies amongst the enemies of one's enemies, etc. Some of it was new to her, so she knew she would have to read the book many times to fully grasp it.
Nesta notices the book advises to command soldiers from the rear, something that Cassian doesn't do, he fights on the front lines with his soldiers.
- A Court of Silver Flames: Chapter 24