Here is the site for free children's eBooks in ePUB format. These were written for FBReader on the XO platform but should work fine on other devices supporting ePUB like the Sony. These are Project Gutenberg books.
These include:
Little Bo-Peep by Leslie Brooke
Aladdin or The Wonderful Lamp by Anonymous
The Three Bears by Anonymous
Miquette baptise sa poupée by Luis de Camões
L'escapade du Miquette by Luis de Camões
More Russian Picture Tales by Valery Carrick
R. Caldecott's First Collection of Pictures and Songs
Nonsense Drolleries by Edward Lear
Little White Barbara by Eleanor S. March
The Royal Picture Alphabet by John Leighton
The Old Man's Bag by T. W. H. Crosland
The Crooked Man and Other Rhymes by Anonymous
Dame Wonder's Picture Alphabet by Anonymous
The Sleeping Beauty Picture Book by Anonymous
The Chickens of Fowl Farm by Lena E. Barksdale
A Masque of Days by Charles Lamb
Enjoy,
Dale