The house of Arden - E. Nesbit

The famous Arden family treasure has been missing for generations, and the last members of the Arden line, Edred, Elfrida, and their Aunt Edith, have nothing to their names but the crumbling castle they live in. Just before his tenth birthday, Edred inherits the title of Lord Arden; he also learns that the missing fortune will be his if-and only if-he can find it before the turns ten. With no time to lose, Edred and Elfrida secure the help of a magical talking creature, the temperamental Mouldiwarp, who leads them on a treasure hunt through the ages. Together, brother and sister visit some of the most thrilling periods of history and test their wits against real witches, highwaymen, and renegades. They find plenty of adventure, but will they find the treasure before Edred's birthday?