
Four Afoot- Barbour-
D. Appleton 1906
I have collected series books (among other types of books) seemingly forever. Yet I never really though much about the book covers. That is, who designed them. Really not even a blip on my screen.
Then I came into contact with an expert in that field, David Ogle. David is researching the Decorative Designers who coincidentally designed and drew numerous series book covers. The information about the Decorative Designers comes from David.

Donald Kirk-Woolley
Little, Brown and Company
1912
The Decorative Designers was a firm that was in business between 1895 and 1932. Henry Thayer was the founder and key employees were Emma Thayer, and Jay Chambers. If it is their design work, it can sometimes be determined by characteristic lettering and cover patterns (by experts in the field and certainly not me). But many of their works can be determined by finding their monogram which is an overlapped DD with the second D reversed on the book's cover. This is generally placed in a nondescript location .

Five Months on a Derelict-
Edwin Houston
Griffith and Rowland Press 1908
Interestingly enough a fair number of series books had cover designs done by the Decorative Designers. Overall as per Publishers Bindings Online
(http://bindings.lib.ua.edu/designerbios/decdesign.html) this firm's monogram appeared on more than 25,000 book covers and jackets.

The Young Bridge-Tender
Arthur Winfield
Street and Smith 1902
Here are some examples of covers with the DD. I have included several Stratemeyer books which should interest Stratemeyer expert James Keeline. Of note is that although the Decorative Designers may have done many covers for Hurst, Grosset and Dunlap, Cupples and Leon and Altemus, their logo rarely appears on any of those publishers' books.

The Rival Ocean Divers
Roy Rockwood
Stitt Publishing Company 1905
Look carefully at the covers shown here. If you click to enlarge them, you should be able to find the DD monogram.
In the future I will review other ways to determine whether your book was designed by the Decorative Designers. All clues given now and in the future about the Decorative Designers are the work product of the true expert-David Ogle.
By the way, look at the blog entry from June 18, 2009.
If you click on the Mershon and Chatterton-Peck Bobbsey Twins' covers, you will see the DD monogram on the bottom right.