The short stories can be a great place to start. Have a look at book reviews and blurbs online with your child to find one that you think might suit them.
Then get your child to listen to the samples offered on sites like:
www.audible.co.uk www.audiobooks.com
www.amazon.co.uk iBooks
by pressing the play icon to see if they like the sound of the reader's voice.
Be warned! Be careful they don't accidentally download the 'free' trial from audible.
Some of these books are very challenging so build up to them rather than plunging straight in (perhaps by starting with ones I've suggested for lower age groups). Also these age guides are just that - a guide! If your 7-year-old can handle Sherlock Holmes then great, go for it. I'm a big believer that children can handle advanced ideas/themes/images when they are presented on the page, in a way that they can't as easily if they see things on a screen. The brain is very clever and a child's mind will instinctively soften the mental picture that words conjure up if they can't handle the extremeness of something. Once a concrete visual image (a scene from a film or something horrific online, for example) has been planted in a child's mind, it is much harder to wipe.
Starter List for Years 1, 2 and 3
SHORT STORIES
The Enchanted Wood by Enid Blyton
Atticus the Storyteller by Lucy Coates
The Selfish Giant by Oscar Wilde
The Bippolo Seed by Dr Seuss
Aesop's Fables (adapted by Michael Morpurgo)
NOVELS
Horrid Henry by Francesca Simon
How to Train your Dragon by Cressida Cowell
Anything by Roald Dahl
Winnie-the-Pooh by AA Milne
The Wind in the Willows by Kenneth Grahame
A Bear Called Paddington by Michael Bond
The Owl who was Afraid of the Dark/The Aardvark who wasn't Sure by Jill Tomlinson
Starter List for Years 3,4 and 5
SHORT STORIES
Rikki-Tikki-Tavi by Rudyard Kipling
Tales from the Arabian Nights by Andrew Lang
Tales of King Arthur by Andrew Lang
NOVELS
Which Witch by Eva Ibbotson
Harry Potter (obviously) by JK Rowling
The Lion, the Witch and the Wardrobe by CS Lewis
Just William by Richmal Crompton
Peter Pan by JM Barrie
The Hobbit by JRR Tolkien
War Horse by Michael Morpurgo
How to Eat Fried Worms by Thomas Rockwell
The Sheep-Pig by Dick King Smith
Charlotte's Web by EB White
The Explorer by Katherine Rundell
Starter List for Years 6, 7 and 8
SHORT STORIES
The Illustrated Man by Ray Bradbury
Someone Like You by Roald Dahl
Splendid Adventure Stories (Various Authors)
Beautiful Stories from Shakespeare by Edith Nesbit
Stories of Sherlock Holmes by Arthur Conan Doyle
The Birds and Don't Look Now by Daphne du Maurier
NOVELS
Goodnight Mr Tom by Michelle Margorian
Coram Boy by Jamila Gavin
Amazon Adventure/South Sea Adventure by Willard Price
The Sword in the Stone by TH White
The Pearl by John Steinbeck
To Kill a Mockingbird by Harper Lee
My Family and Other Animals by Gerald Durrell
The Railway Children by E Nesbit
Swallows and Amazons by Arthur Ransome
The Hunger Games by Suzanne Collins
Treasure Island by Robert Louis Stevenson
Around the World in 80 Days by Jules Verne
Animal Farm by George Orwell
Moonfleet by J. Meade Falkner
1984 by George Orwell
Gulliver’s Travels by Jonathan Swift
Ballet Shoes by Noel Streatfield
Northern Lights by Philip Pullman
The 39 Steps by John Buchan
The Boy in the Striped Pyjamas by John Boyne
King Solomon’s Mines by Henry Rider Haggard
Lord of the Flies by William Golding
Jane Eyre by Charlotte Bronte
Brave New World by Aldous Huxley
The Colour of Magic by Terry Pratchett
The Woman in White by Wilkie Collins
War of the Worlds by HG Wells
Noughts and Crosses by Malorie Blackman
Green Hills of Africa by Ernest Hemingway
Great Expectations by Charles Dickens
White Fang by Jack London
Uglies by Scott Westerfield
The Lord of the Rings by JRR Tolkien
The Siege of Krishnapur by JG Farell
Stone Cold by Robert Swindells
Plus anything else that takes your fancy (especially if it was written more than 40 years ago - in fact the longer ago the better!)