Da Vinci's Cat, by Catherine Gilbert Murdock

A boy from 16th century Rome and a girl from 21st century New Jersey discover a time machine and accidentally set off a chain of events which could rewrite the history of the Italian Renaissance.  

Young Federico, kept hostage by the Pope, is on friendly terms with the artist Raphael and familiar with the far grumpier Michelangelo.  They are fine artists but he doesn't understand the intense interest in them from strangers wearing strange clothes who appear out of a closet in the Palace.  These strangers are desperate to acquire cast offs from Raphael and Michelangelo.  Even rough sketches excite them (especially if they are signed)!  And in return, they bring Federico the most delicious sweets, made from something called "chocolate." Federico has never tasted anything better in his life and sees no harm in absconding with a drawing or two in exchange for more of it.  But when Bee shows up, history is changed and only the cat seems to understand it all, and she doesn't say much.

An amusing time travel story with some educational value from the attention it gives to the artistry of Raphael and Michelangelo and to its many lovely details about everyday life in 16th century Italy.  The story doesn't always pace well and some parts get confusing (as is true with many time-travel stories!), but it is entertaining.  Absolutely lovely sketches of the cat Juno throughout the novel are a high point.