Originally Posted by Menshevik
I'm gonna assume you wanted a book about medieval warfare, being that your post is in the Medieval history forum. Barbara Tuchman's , A Distant Mirror, is one of my all time favorite books. She writes history like no else, I just hope a 13yr old can appreciate it. Also in John Keegan's, The Face of Battle, he devotes the first portion of his book to the battle of Agincourt. He describes beautifully and intricately what Medieval warfare was like.
The Tuchman book is an excellent suggestion, but as you mention, I don't know if the work can be appreciated (or understood) by a 13 year old.
Here are a few that I can recommend that might be at a teen level:
[ame="http://www.amazon.com/Art-Catapult-Ballistae-Trebuchets-Artillery/dp/1556525265/ref=sr_1_7?s=books&ie=UTF8&qid=1356819801&sr=1-7&keywords=teens+warfare"]Amazon.com: The Art of the Catapult: Build Greek Ballistae, Roman Onagers, English Trebuchets, and More Ancient Artillery (9781556525261): William Gurstelle: Books@@AMEPARAM@@http://ecx.images-amazon.com/images/I/51phcoM8sQL.@@AMEPARAM@@51phcoM8sQL[/ame]
[ame="http://www.amazon.com/Medieval-Warfare-World-Crabtree-Paperback/dp/0778713768/ref=sr_1_23?s=books&ie=UTF8&qid=1356819998&sr=1-23&keywords=teens+warfare"]Medieval Warfare (Medieval World (Crabtree Paperback)): Tara Steele: 9780778713760: Amazon.com: Books@@AMEPARAM@@http://ecx.images-amazon.com/images/I/511M47S4M6L.@@AMEPARAM@@511M47S4M6L[/ame]
[ame="http://www.amazon.com/Boys-Book-Battles-Illustrated-Press/dp/1409938271/ref=sr_1_70?s=books&ie=UTF8&qid=1356820125&sr=1-70&keywords=teens+warfare"]The Boy's Book of Battles (Illustrated Edition) (Dodo Press): Eric Wood: 9781409938279: Amazon.com: Books@@AMEPARAM@@http://ecx.images-amazon.com/images/I/51Fd9TfIiZL.@@AMEPARAM@@51Fd9TfIiZL[/ame]
The last book isn't from the medieval period, but these sorts of books he might find of interest.
Here's one historical fiction that every teen should read:
[ame="http://www.amazon.com/Brother-Dead-James-Lincoln-Collier/dp/0439783607/ref=sr_1_1?s=books&ie=UTF8&qid=1356820218&sr=1-1&keywords=my+brother+sam+is+dead"]My Brother Sam Is Dead: James Lincoln Collier: 9780439783606: Amazon.com: Books@@AMEPARAM@@http://ecx.images-amazon.com/images/I/51kFCYyrMpL.@@AMEPARAM@@51kFCYyrMpL[/ame]
It's an excellent book and it really gets kids interested in history.