The Mystery

From Author Wars - https://authorwars.com/publications/the-mystery-506445.html

Overview:

Title: The Mystery
Author: Samuel Hopkins Adams, Stewart Edward White
Year: 2013
ISBN-10: 9781494379605
Publisher: CreateSpace
Price: $7.55
Pages: 130
Binding: Trade Paperback
Type: Novel
Title Reference: The Mystery
Notes:
Data from Amazon.com as of 2015-03-10.

Cover illustration is that of the plate facing page 14 in the 1907 McClure, Phillips & Co. edition, with credited illustrations by Will Crawford (viewed at HathiTrust).