Levy's new book. It is really good for beginners. How to win at chess by Levy Rozman.
What is the best book to buy for a 6 year old beginner?

A six year old reading chess books? That is impressive!
I don't know if he will read it, but I know that he won't read it if I don't buy him it.

I am thinking of buying a 6 year old a chess set and a chess book for Christmas.
He has never played chess before, so it needs to be a fairly simple and beginner friendly book.
What do you think would be best for him?
"Chess for Children" by Murray Chandler is one I'd recommend checking out. It's for beginners younger than 10.
It teaches the setup, the pieces, and how they move and capture. Lots of arrows and large, easy-to-see diagrams. It also has quizzes along the way, like "What's the name of this piece?" or "How does this piece move, again?"

"Stepping Stone 1" and "Stepping Stone 2" from the Steps Method are designed for 4-8 yos: https://m.chess-steps.eu/books-stepping-stones.php
Kids take more interest when presented with visual illustrations, which makes me pick these two titles for a 6-year-old. "Chess for Kids by Michael Basman" & "Let’s Play Chess: A Step by Step Guide for Beginners by Bruce Pandolfini" both use pictures with simple explanations that kids would love to read through and learn the game.
A 6-year-old would need a senior guide to understand things. So, this is one that I believe can be a good pick. Chess is Child's Play by Laura Sherman and Bill Kilpatrick uses an approach for a parent to guide their kids and is interactive enough for kids to stay focused and interested.
Michael Basman's Chess for Kids is indeed a great pick for its approach to teaching the history, fundamentals, and strategies to kids

It is only about 44 pages long, and like all DK books, those pages are mostly nice photographs. I think Chernev's and Seirawan's books would provide much more value if you don't need the pictures to motivate you.

A 6-year-old would need a senior guide to understand things. So, this is one that I believe can be a good pick. Chess is Child's Play by Laura Sherman and Bill Kilpatrick uses an approach for a parent to guide their kids and is interactive enough for kids to stay focused and interested.
i found this senior guide that might work and is based on a series of 2 hour lessons
Chess Course in 12 Lessons for absolute Beginners, seniors. Advice for their Coaches. Never too late!: Learn to play honorably from Scratch: basic Tactics ... basic Ending, winning Moves (Chess manual) Rodolfo Pardi
Most serious teaching methods are aimed towards young and very young students, or to players who wish to become Masters. The multitude of chess books published over the years have not taken senior citizens into consideration. This method instead is adjusted to suit the capabilities of the elderly, taking into account their tendency to be more easily distracted and less focused than a child, and also because of short term memory problems.

Thanks everyone for your help and advice. I ordered two books, a second hand copy of the hardcopy Dorling Kindersley book by Micheal Basman, and Chess for Children by Murray Chandler.
I spent a few minutes with him last weekend teaching him how the pieces move. He is around 100 Elo level at the moment, but after Christmas, if he gets through both books and is improving, I think I'll buy him the Levy Rozman book as a follow up.

Teacher here. Some of the suggestions here are terrible for a 6 year old kid. Separate the person from the book and you will find the Rozman book will be a wate of money. It's not engaging for little kids.

Chess Courses - Instructional Resources...
https://www.chess.com/blog/RussBell/beginners-chess-course-instructional-resources
I am thinking of buying a 6 year old a chess set and a chess book for Christmas.
He has never played chess before, so it needs to be a fairly simple and beginner friendly book.
What do you think would be best for him?