THE MYSTERIOUS AFFAIR AT STYLES

The full desription on the title page reads: The Mysterious Affair at Styles. A Detective Story.

The title page also has a further print: This book, while produced under wartime conditions, in full compliance with government regulations for the conservation of paper and other essential materials, is COMPLETE AND UNABRIDGED.

Red covers with sunning to the spine and top front edge. 296 pages. Clean and unmarked inside.

Published by Grosset & Dunlap, New York. The Copyright page reads: Copyright, 1920. By Dodd, Mead & Company, Inc. Printed in U.S.A.

Various resources differ on the true first edition of this first novel by Christie. 

(Agatha Christie- A Reader's Companion by Wagstaff & Poole - 2004 page 19) mentions: "The first UK edition is published in January 1921 following serialization in the Weekly Times. The American edition preceded it, published in December 1920. 

The Mysterious Affair at Styles is the first detective novel by British writer Agatha Christie, introducing her fictional detective Hercule Poirot. It was written in the middle of the First World War, in 1916, and first published by John Lane in the United States in October 1920 and in the United Kingdom by The Bodley Head (John Lane's UK company) on 21 January 1921. (wikipedia)

Christies mentions some more detail. https://www.collectingchristie.com/post/1920s_agatha_christie_grossets