R. M. Ballantyne 
Post Haste 

In this fun adventure in merry, old England, the reader is taken back to the beginning days of the General Post-Office, learning about the founding of the mail system. In the 1800’s, guaranteeing a letter to delivery was a bit of a challenge, as Phillip Maylands, his sister Mary, and their friend George Aspel quickly find out. Though the British Post-Office at this time delivered in good condition over fourteen hundred million letters, a half-dozen important ones addressed to Messr. Blurt and Co. are missing! Our friends must take on the role of detective to trace the mysterious disappearance. (Goodreads)