Heritage Auctions now has the following letter up for auction:

Major General Samuel R. Curtis Autograph Letter Signed About the Death of His Son Killed During a Raid by William Quantrill. Three pages of a bifolium, 7.75" x 10". Department of Kansas & Indian Territory; October 31, 1863. A letter to Brigadier General William K. Strong thanking him for his condolences and relaying the details of the death of his son, Henry Z. Curtis. Major Henry Curtis was killed at Baxter Springs, Kansas, while travelling with General James Blunt. The company was ambushed and outnumbered by a band of between 250-500 Confederate guerillas led by Quantrill. Samuel Curtis describes the events in his letter as follows:

"... The details have probably reached you by the papers. The attack on General Blunt's Escort was sudden and the work short. The Rebels took no prisoners or shot them soon after they were taken. 83 bodies have been found. Henry's horse was shot and fell, but followed our men and escaped leaving Henry whose body was found near the spot where the horse fell. The powder marks, and scorched whiskers showed that the weapon had been held close to his head. He must have died instantly. They took his coat, boots, sword and money, but left his watch which was in his fob without a chain, and his sleeve buttons and a ring on his finger. His pistol was also found near by: five of the six loads discharged. He evidently fought and fell fighting for his General and his Country. We sent you a news paper which I hope you received giving the details of the final sad duties which were paid to his body when it arrived here. We are almost broken hearted. The loss and the circumstances are all heart-rending. My poor wife and his young widow, are trying to bear their affliction, and we all try to be reconciled to all our terrible ills..."

According to military records, the attack at Baxter Springs was supposedly the only official report that Quantrill ever submitted.*

You can see the letter at this link:

https://historical.ha.com/itm/autographs/major-general-samuel-r-curtis-autograph-letter-signed-about-the-death-of-his-son-killed-during-a-raid-by-william-quantrill/a/6196-47026.s?ic4=GalleryView-Thumbnail-071515

Or just go to Heritage Auctions and search for it.