
I had to laugh when I read this because I knew my child could never pick one favorite book. Is there such thing as one favorite book? We've never heard of such a thing.
One book? LOL As if! She's been a book junkie since before she could talk. Here she is in K rolling in books. LOL

I asked her anyway. Her answer was "The Percy Jackson series
In the interest of being helpful I'll list some of the books that she rereads most often.
Usborne Time Traveler
Usborne Living Long Ago
A Little Princess
Understood Betsy
Strawberry Girl
Twenty and Ten
Witch of Blackbird Pond
The Cricket in Times Square
Mr. Revere and I
My Father's Dragon
Ben Franklin of Old Philidelphia
That should get you started. Happy Reading!