The Case of the Missing Marquess:An Enola Holmes Mystery by Nancy Springer

Enola Holmes wakes up on her 14th birthday to find her mother missing. Her two older brothers, Sherlock and Mycroft, find clues that suggest Mrs. Holmes has run off and plan to send Enola to boarding school. Enola deciphers clues left for her by her mother and decides to run away herself. On her journey, she gets involved in another mystery involving the missing Marquess of Basilwether, a young boy who's parents fear has been kidnapped.

Stormbreaker by Anthony Horowitz

"...a car accident...called the ambulance...intensive care...nothing anyone could do...so sorry." The voices of the police at the door at three in the morning drift up the stairs to Alex as he listens from his bedroom door. Ian Rider, his uncle and the only parent he ever knew, is dead. But circumstances of his death don't sit right with Alex. Ian Rider, a banker, always wore his seat belt and he made sure Alex wore his; but he wasn't wearing one the night of the accident according to the police. Then when Alex returns from the funeral to see a Stryker & Son moving van pulling out of their home and discovers Ian's upstairs office is empty, Alex decides to do some investigating on his own. He locates Ian Rider's BMW at J. B. Stryker Auto Wreckers and discovers the driver's side has been riddled with bullets. After narrowly escaping danger in the wrecking yard, Alex is summoned to the Royal & General Bank, where Alex learns that Ian Rider was not a banker but a spy for the British Intelligence Agency, MI6. Ian was undercover on a case investigating the release of the Stormbreaker computer when he was killed and MI6 needs Alex Rider's help to finish the job.