Herb Mott drew the cover and internal illustrations for "Deadly Double," which was also the last book in the series to have any internal illustrations. The cover art for this book was reasonably good, but the internal illustrations were not up to the standards set by the earlier artists. The boys are drawn almost as caricatures, and the scenes that were selected for illustration were not that memorable in general. Overall, this was not a fitting end to the era when the Investigators' books were illustrated.
The reasons for the poor artwork in this book are anyone's guess. Other artwork by Herb Mott is vibrant, colorful, and quite good. It is my opinion based on inspection of the artwork in the book that there may have been a breakdown in negotiating a timeline or payment for the artwork; only one of the illustrations looks finished. The other drawings appear to be rough drafts, and this may explain the raw quality of them.