The Mysterious Benedict Society and the Prisoner's Dilemma by Trenton Lee Stewart: There's enough mystery in The Mysterious Benedict Society series to qualify it for Mystery Monday, though it's also fraught with adventure and fantasy. The extraordinary group of children assembled by Mr. Benedict in the first book, The Mysterious Benedict Society, return for their third adventure. Villainous Ledroptha Curtain is trying to reclaim the dangerous Whisperer for his own nefarious purposes, and Reynie, Sticky, Kate, and Constance are in seclusion for their own protection. The opening Prisoner's Dilemma exercise with the children is hilarious, and the unfolding adventure is rollicking as usual, with added suspense regarding Constance's new powers and her mysterious past. This one feels like the end of the series, though the door is open for future adventures. Start with the first book.
Source disclosure: I purchased this book.
Enola Holmes and the Missing Marquess by Nancy Springer: Enola Holmes is the much younger sister of Sherlock and Mycroft. She lives alone with her mother, who disappears on Enola's fourteenth birthday. She enlists the help of her brothers, who are shocked at the decrepit state of the family estate and at Enola's total lack of femininity and polish. They plan to send her to finishing school, but Enola has no desire to be a proper Victorian lady. She dresses as a widow and escapes to London, where she becomes involved in the disappearance of a marquess while eluding her brothers. Enola is thoroughly engaging, a plucky, independent heroine trying to escape the constrained life of a Victorian lady. The mysteries (of her mother and of the marquess) are interesting and complex and the addition of Sherlock and Mycroft is great fun for those familiar with Sherlock's tales. Victorian England is beautifully imagined, in a realistically squalid way, and the language of flowers and ciphers of which her mother was fond are nice touches. I also read the next three in the series: The Case of the Left-Handed Lady, The Case of the Bizarre Bouquets, and The Case of the Peculiar Pink Fan, in which Enola solves more mysteries while eluding her brothers. This is a delightful series.
Source disclosure: I purchased Missing Marquess, Left-Handed Lady, and Bizarre Bouquets. I received Peculiar Pink Fan through BookMooch.