Above: I love the deep greens and rolling hills in this illustrated dust jacket for Chard Powers Smith's 1939 novel "Artillery of Time." The weighty tome tells the tale of a New York state farmer in the 1850s and 1860s dealing with the country's dual marches toward industrialization and the Civil War.
(Alas, the Motor Girls appear to have been less popular than the Motor Boys.)
The listed author is Margaret Penrose, but that was a Stratemeyer Syndicate house pseudonym used for several different series.
Many of the "Margaret Penrose" books were written by Lilian Garis, the wife of Howard R. Garis (of Uncle Wiggily fame).
Here's a final closeup of the Motor Girls as they motor off on an adventure...