The book is the first in a Sherlock Holmes series called Young Sherlock Holmes. The first book is called "A Study in Scarlet" and is a younger reader's version of the original Sherlock Holmes book by the same name. A plot summary can be found here: http://en.wikipedia.org/wiki/A_Stud The full title should read: Young Sherlock Holmes: A Study in Scarletbook and should indicate somewhere that it's book one in the series. It takes place in the 1880s, and the designs should reflect that period.
The book is aimed at readers in the 11 to 13 year old range. Sherlock is a bit younger than in the normal works, in his early to mid twenties. Watson is also younger, perhaps in mid to late twenties. The feel of the cover design is going to be key in dictating the age of the readers, so it's important that it appeal to that age group.
Some elements from the story that are possible things to work into the design, but not necessary: 1) a dead body found poisoned, 2) the murderer wrote the word "Rache" on the wall in blood, using his finger, 3) the murderer was the driver of a hansom cab, so the cab could be used, 4) a box holding two identical pills, one which is poisonous, one of which was not (the pills are small, round, and gray), 5) a gold wedding ring, used to remind the murdered man of his wrongs, 6) the murderer as a younger man with a rifle in the Utah rockies, 7) and of course, Sherlock himself (with or without watson)
The book is going to be distributed largely through Amazon and other online sources, so the thumbnail is the most important thing. I will want a thumbnail to use that will be a square, even though for the printed book the shape will be a rectangle. Thus, the image generated should be square and when printed it will wrap around to the back, though will only cover a bit of the back. This also means that anything of importance in the image should not be on the far left side or else it will be lost in the physical book. The importance of the thumbnail also means that anything done in fine detail will be lost. There can be fine details, but the central image must look good as a thumbnail.
The two authors are Sir Arthur Conan Doyle and Leo Zanav. It should say something like: The Classic Story of Sir Arthur Conan Doyle as retold by Leo Zanav.