My son (age 6) loves all the Dragon books by Cressida Cowell (How to train your dragon/How to be a Pirate/How to speak Dragonese etc)
They are great, funny and good vocabulary and I don't mind reading bits to him from time to time.
The other books he adores and simply pores over and which I find tedious are the Club Penguin range. He loves the ones that you can choose your own path (The Inventor's Apprentice/Stowaway/Star Reporter)but he'll ready anything with a Club Penguin logo on it.
We both like all the Paddington books.
He recently brought home a very battered looking copy of 'The Better Brown Stories' by Alan Ahlberg which I think is fairly advanced for a 6-year old so I'm reading it to him as a bedtime story. It's a unusual story, about a family called the Browns who
get fed up with the boring stories their 'writer' writes about them, so they demand more excitement. Each story is the writer's attempt to spice up their lives, it's an eccentric sort of book.
I second the suggestion of Stig of the Dump - I hated it as a child, but maybe it's a boy thing, my son adored it.
The other set of books that are totally brilliant are the Dragon Detective Agency series. I read them to my son at 5, not sure if he'd tackle them himself - possibly. But a 7/8/9 year old would love them, equally good for girls and boys. By Gareth P Jones. The first book is called 'The case of the missing cats.'