The book can be read at the Internet Archive.
Per Wikipedia: Miss Lavinia Pickerell is an unlikely heroine: prim, spinsterly, angular and stiff, wearing old‑fashioned clothes and an outlandish hat, and devoted to her pet cat, she nevertheless manages to inadvertently stow away on a rocket to Mars in her first adventure. But with her common-sense, practicality, and unflappable demeanor she manages to bring each adventure to a satisfactory conclusion. She is every child’s favorite maiden aunt, and is possibly modeled after MacGregor herself.
Photo by Stellar.