DS2, who is 5, adores the Dinosaur Cove books by Rex Stone.
He is obsessed with dinosaurs.
The books are about two boys who discover a cave through which they are able to go back in time to the land of the dinosaurs. There are a lot of books in the series and in each one they have an adventure where they investigate a different type of dinosaur.
According to DS2, who is more of an expert on the subject than I am, the author gets his facts right.
DS2 is not that keen on fiction - loves fact books, mainly about dinosaurs (do you see a theme emerging), but is happy to read these books to himself. They have pictures on each page, but are chapter books.
I read the first few to him, but when I agreed to buy him one when we were in Waterstones last weeek I made it conditionally on his trying to read it himself.
He finished it in the car before we got home (Miracle!)