I bought this at the Museum of London gift shop to stop my 6 yr old son pestering me for tat! I was surprised to find it kept him engrossed for the hour's journey home and he's read it a couple of times again since.
Essentially it's a bit like Horrible Histories for younger children, with less focus on the gruesome stuff but still lots of fun facts and funny pictures. It's written as though the reader is Samuel Pepys and explains what you'd be experiencing during the Fire of London, with lots of interesting details about life in 1666 London. It's well-designed with detailed and entertaining colour illustrations and the text is in manageable chunks with lots of boxes, thought bubbles, etc.
The "Avoid" theme is quite clever as it helps to engage the reader, getting them to think more critically about what kind of things were done to fight or prevent fire.
I'm about to buy some of the others in the series at my son's request. Although he likes to read fiction he'd much rather be playing on his DS, so I am impressed to find a non-fiction series he enjoys reading. My son is quite a confident reader for age 6 but he had to ask me about some of the words so I'd say it is suitable for approx 6-8 yr olds.
If the publisher reads this, please try to get the "look inside" feature onto some of the books in this series as it's very hard, as a parent, to judge whether a book will be appealing for your child, or is the right reading age without being able to see a page of it.