My sons were still quite small when the first Horrible History books (The Terrible Tudors and The Awesome Egyptians) were published. Which was great because that way there were a few around by the time they started reading and they could read a lot of them while growing up. They both love history and I think this is partly due to these books.
The subtitles of most of these books are "History with the nasty bits left in!" Boys, especially boys, love a bit of gruesomeness, some blood spilled in the books and since most of history has usually both of those parts, it's easy to get them there.
The language is just right for kids. For example, in "The Awful Egyptians", the chapter "Phunny Pharaohs" (yes, they also play with words in a fantastic way), it is mentioned that the awesome Egyptians lived in the land by the Nile: "Every year the river flooded and left lovely squidgy mud covering the fields ... lovely squidgy mud that was good for growing crops. ..." Not only can the kids squirm at the thought of the squidgy mud, they also learn that the mud is god for something.
There are a lot of books about all sorts of history, from the Egyptians to the Aztecs, the Middle Ages to the World Wars in the 20th century, from Kings and Queens to the United States, there is something there for everyone. But, be warned, once the kids have read one book, they will become addicted and won't stop reading until they've seen them all.
This is a list of the history books. There are now also science (Horrible Science), geography (Horrible Geography) and math (Murderous Math) books available.
Barmy British Empire
Cruel Crime and Painful Punishment
Cruel Kings and Mean Queens (The Kings and Queens of England, Britain, and the United Kingdom)
Cut Throat Celts
Dark Knights and Dingy Castles (The history of Knights and Castles)
Frightful First World War
Horrible History of Britain and Ireland
Horrible History of the World
Measly Middle Ages
Savage Stone Age
The Twentieth Century
Wicked Words (History of the English language)
Wild Warriors (Vikings, Celts, Huns and Samurai)
Woeful Second World War
From the back cover of "England" (one of many examples): "History with the nasty bits left in! The Horrible History of England reveals the awful truth behind the rebellions, riots and rumpuses that have made England what it is today (whatever that is). From the cruel Celts right up to the terrible 20th century it's a tale that will make you quake. Want to know: - which monk tried to pinch the devil's nose with a pair of tongs? - why some people in the Middle Ages ate dove droppings? - which English king was accused of being a werewolf? Whether it's plague, fire, and civil war, or roast beef, fish and chips and a cup of tea, if it's got anything to do with the English it's in this book. Mull over the miseries of Middle Ages monarchs, discover ten ways to be killed down a Victorian mine, and find out which dreadful days in history to celebrate with a day off school. History has never been so horrible!"