There are quite a few cracking classic children's books around, although some are quite hard to read! Some have already been mentioned, but what about Pippi Longstocking, A Little Princess, Ballet Shoes and White Boots, The Phantom Tollbooth, Anne of Green Gables, Charlotte Sometimes, Charlotte's Web, Tuck Everlasting, Pollyanna, Heidi, Treasure Island, Just William, Alice's Adventures in Wonderland and Through the Looking Glass?
Some contemporary books are also fantastic reads. I would definitely recommend:
Skellig by David Almond;
My Sister Lives on the Mantelpiece by Annabel Pitcher;
Rooftoppers and Girl Savage by Katherine Rundell;
Goodnight Mister Tom and Back Home by Michelle Magorian...
Older children might be more interested in The Machine Gunners, The Boy in the Striped Pyjamas and Carmen Reid's "Cross My Heart".
I would also recommend anything by the likes of Eva Ibbotson, Hilary McKay, David Walliams and Roald Dahl! :)