25 of the original Nancy Drew books were illustrated by commercial illustrator, Russell H. Tandy from 1930 to 1949. These include the original art dust jackets for volumes 1-10, 12-26 as well as the internal illustrations for volumes 1-26. The original cover for book #11 is a mystery--likely not Tandy.
Tandy had been illustrating some of the books for Edward Stratemeyer's Books for Girls series. It was Stratemeyer, in the planning of his soon to be published Nancy Drew series, who submitted the cover illustration of two artists to Grosset & Dunlap: Ernest Townsend's dust jacket art for Two Girls and a Mystery, 1928, and Tandy's dust jacket art for Kate Martin's Problem, 1929. Both covers are shown below.
Grosset & Dunlap chose Tandy as the illustrator for the Nancys--a wise choice indeed, for it was Tandy's classic and sophisticated portrayal of Nancy that helped make the series sell quite successfully. Nancy below and Kate above are very similar.