Hi there
My older son (now 7) is a good reader too, and I agree there is a shortage of material for that young reader stage. Once your child gets to 6 years old it gets a lot easier. At the moment try short little books that can be read in one evening. From 6 yeasr, go onto books with several chapters and start losing the illustrations.
We have found the following really good (but many of these apply to 6-7 years):
Magic Tree House Series. Fairly easy. (Brother and sister, Jack and Annie go to a Tree House, every time they go they look in a different book and get transported to a different adventure, with some common themes running through the books). Mary Pope Osborne
Rascal the dinosaur series. Easy.(Paul Jennings?) These have very simple text about a pet dinosaur and his family adventures. The pictures look like they've been clipped from computer games - modern kids love these - but the stories have traditional themes and language.
Peter and Jane Ladybird Series. Easy.. progressively harder. Slightly stilted language but good for early reading skills.
Tashi series. 6+ years. Anna Fienberg. Stories about an elf like character who becomes part of a family. He tells stories of his life's experiences to the family's son and these form the content of the book (slight fantasy feel with a clever mini cliffhanger on every page to keep the pages turning).
Captain Underpants series. 6+ years. Dav Pilkey. Boys love this - silly stories and jokes about toilets, schoolboys and a headmaster who transforms into a Y-fronts wearing superhero.
Charlie Small series. 7+ years - absolutely brilliant series about a 400-year-old 8 year old boy (!) who has many adventures in far flung lands. My son will actually read these for an entire day without moving.
Zac Power series - 7+ years. Sci fi hero kid type stories that get read quickly.
Secret Seven and Famous Five. Enid Blyton. My son loves these , but due to the stilted language and long plot lines, he needs them to be read to him. There is a brilliant recent dramatisation of these stories on CDs (free in Daily Telegraph - might be able to buy them?).
Roald Dahl - 6+ years. Charlie and the Chocolate Factory etc. Still absolutely brilliant books.
I don't recommend Horrid Henry series - my child loved these books, but they gradually converted him into some horrid behaviour. Children just don't get the irony ... they just copy.
Happy reading.
The library gets really useful at 7+ years. It's just too expensive to buy 2 novels a week if you have an avid reader!