Monday with a Mad Genius by Mary Pope Osborne (Magic Tree House Series #38)

Merlin the magician is in trouble, and he won't eat or sleep or speak to anyone in Camelot. In the magical world of this best-selling series, it's the four secrets of happiness that are missing from Merlin's life, and Morgan Le Fay sends Jack and Annie on a mission to find them. This time, they find themselves in Italy during the Renaissance, when every new morning brought the promise of artistic and scientific discovery - especially for a dreamer, artist, and inventor like Leonardo da Vinci. What secret can he give the kids to bring back to Merlin?

Two kids zoom through time and around the world to be on the scene at key times in history. Famous characters, both real (like Isaac Newton) and fictional (like Merlin the magician), plus well-described settings (like the World's Fair in Paris in 1889) add to the fascinating fun. The series features accurate historical facts in exciting plots.

Goodreads: 4.08

Hardcover, 2007