For my native speaking 6 year old, hits so far have included Harry Potter; Enid Blyton - the Wishing Chair series, the Faraway Tree series; Tom's Midnight Garden; Cressida Cowells How To Train Your Dragon series; Charlottes Web; Mrs Frisby and the Rats of Nimh; Stig Of The Dump.
For non-native speakers, I would go long with Biff - so maybe pick books for slightly younger children which are not too wordy. For example the Allan Ahlberg books like Each Peach Pear Plum, the Red Nose series, the Jolly Postman series. Also you have the Ladybird Well Loved Tales books have traditional fairy tales told simply and well illustrated. Also, Peter Yarrow of Peter Paul and Mary fame made a book of Puff The Magic Dragon which comes with a CD with the song on it - the illustrations in the book are just breathtaking.
My just turned 7 year old is currently wrapped in the Narnia series by C.S.Lewis. He has The Twits and The Witches by Roald Dahl lined up next. He loves reading Dr Suess books too - over and over again.
they may be a little young but I have to second Nickinbasels recommendation of the 'how to train your dragon' series. My two read and re-read those until the books fell apart.
Also they liked the Mr Men, Thomas the tank engine, anything to do with dinosaurs (Harry and the Dinosaur series was good) and also any type of factual book with good illustrations and simple text. Boys often seem to read non fiction in preference to fiction and collect an enormous amount of odd knowledge and facts and figures along the way.