This is a book my mother got when she was a teenager, and had passed down to me. That copy of the book was lost in a move, alas, but I remembered it fondly and recently tracked it down again.
The Shirley in this book is in her mid to late teens, and is as level headed and caring as any child she ever played. She is thrown into her friend's family conflict and thus is somewhat on the edge of things, but Shirley manages to have some adventures of her own even so.
While there is a mystery at the center of the book -- why did Betha's grandmother disappear, and where has she gone? -- it isn't a mystery in the sense of the main characters tracking down clues and figuring out what is going on. Shirley does run across a number of clues and otherwise collects information, but generally they just add to her bewilderment.
Eventually, all is made clear, and justice is done. In that sense, the inside cover illustration is a spoiler. But the illustration doesn't answer either of the questions that drive the book, nor does it answer other questions that arise out of the plot, so while it serves to reassure the reader of a happy ending, it doesn't spoil the book in any meaningful way. The book came out during WW II; that kind of spoiler was a good thing, IMHO!
The clothes are different and there are no cell phones, but the plot has a timeless quality and the motivations and the beliefs of the villain of the piece, while perhaps less common, are certainly still held by many, alas. Not a great classic of a book, but involving and entertaining and a pleasant evening read.