Buckley-Archer, Linda (2006). Gideon, the Cutpurse. 404 pages. Grades 5-8.
Peter has been looking forward to spending his twelfth-birthday with his father, who has had to postpone their plans twice already. When his birthday is put on hold for the third time, Peter has had enough. His father sends him to Derbyshire for the weekend with his au pair where he meets Kate and her large, but loving family. Peter imagined the weekend as being a horrible time in the country, but he couldn't be more wrong. During a trip to Kate's father's science lab, Peter and Kate stumble upon a time-travel machine, known as the anti-gravity machine, which transports them back to London c. 1763. At first, they are uncertain of where they are and what year it is, until they meet Gideon Seymour who aids them throughout their journey. In order to get back to their time period, they must rely on this stranger who knows where the anti-gravity machine is being held. While stuck in the London of 1763, the children meet King George and Princess Charlotte, as well as a few other historical figures. Find out if Peter and Kate make it back to 21st century London. The adventure continues with the sequel: The Time Thief.
And if the historical detail leaves you wanting to read more about the 18th century, try:
Steve Sheinkin's, King George: What Was His Problem?