Six year olds tend to come across easy reader / emerging reader book series, which are often more about learning phonics, at school.
In terms of reading for pleasure, six year olds who have started to read independently tend to go for short chapter books which have simple language, fast-paced plots and lots of illustrations.
New readers are often keen to read one book after another in a series, spending more time with familiar characters and building their reading confidence that way.
Below are suggestions for popular book series for six year olds:
Dog Man: Twenty Thousand Fleas Under the Sea
by Dav Pilkey
Dog Man (book 11). Graphic novel series in colour for age 6+ about crime-fighting dog man.
Rhinos at Recess
Mary Pope Osborne, AG Ford (illustrator)
Magic Tree House (book 37). This time travel series is a perfect introduction to chapter books for readers age 6 and up. There was a rebranding/renumbering in 2017 and there is now also the Merlin Missions books for more confident readers. In addition, there is a Fact Trackers series.
The Borrowed Puppy
by Holly Webb & Sophy Williams (illustrator)
Animal Stories, an apparently endless series about kittens and puppies for children age 5+.
The 156-Storey Treehouse
by Andy Griffiths & Terry Denton (Illustrator)
The Treehouse series (book 12). Absurd humour about a treehouse which grows 13 storeys between each book in the series. Graphic novel in black and white, suitable for emerging readers with a taste for silliness, age 6+.
Also out recently Treehouse Tales: Too silly to be told… until now!
Narwhalicorn and Jelly
by Ben Clanton
Narwhal and Jelly (book 7). Graphic novel series for readers age 5+ about a happy go lucky narwhal and a no nonsense jellyfish.
Toto the Ninja Cat and the Legend of the Wildcat
by Dermot O’Leary & Nick East (Illustrator)
Toto the Ninja Cat (book 5). The adventures of Toto and her brother Silver are an engaging introduction to chapter books for children age 6+, with plenty of illustrations.