Lucy sounds delightful Some suggestions, from what you've said that she might enjoy and welcome her to the wonderful world of reading for pleasure!:
What about The Enchanted Wood, Enid Blyton? Far Away tree, The Wishing Chair etc?
There are some wonderful Enid Blyton books which you can find on Amazon (second hand). The Five Find Outers are excellent and The Adventure series unsurpassed.
The Secret Series - Moon Castle, Killimoonin, Spiggy Holes are brilliant! The Secret Island was Enid Blyton's eldest daughter's favourite book of all time, wonderful, wonderful, wonderful, yet hardly known.
School stories - Malory Towers, St Clares
The Barney Mysteries - The Rubbadub Mystery, The Rockingdown Mystery etc all great too!
Famous Five and Secret Seven are dull in comparison and 'younger'.
Children's classics? The Little Princess, Secret Garden, Black Beauty, Charlotte Sometimes (Not a classic but should be) A Traveller in Time (Alison Uttley) Serialised in the 70s when I was her age, brilliant, brilliant brilliant!
The Princess and the Goblin (the book that inspired Enid Blyton) George Macdonald (?) From memory? Really interesting read even as an adult!