DS7 loves both the MM and Horrible series. I bought the MM series; I try to pick up the Horrible books at used bookstores or through interlibrary loan. Sometimes we get them sent from my UK-in-laws.
Kjartan Poskitt, the author of the MM series, has a degree in math or physics/engineering, I think. That's probably one reason why the MM series are so solid in terms of math. DS loves them and keeps going back to them though he's read the entire series. I think he's learned a lot from them without letting on.
The Horrible Histories series, by comparison, is written by Terry Deary and he doesn't have a degree in history. He comes from a theater background. As a result, Deary relies on researchers for the series. While the history is accurate, I have to turn the other way with some of it since I was studying for a PhD in 17th-century history (UK/New England) before I had DS. The facts are accurate and the material is presented in an engaging Capt Underpants-type way, but there's definitely some oversimplification in areas. But that's ok with me.
I have to say that DS7 absolutely loves the HH series. I cannot keep DS7 away from the HH series and it has served as a gateway to studying time periods or peoples/places further. He's since been reading about the Aztecs and Incas, which he probably wouldn't have done so otherwise. DS loves the HH videos (on YouTube) and has pick up tons of general knowledge from them. Beware that bathroom humour features strongly and the chopping block talk (think Henry VIII). There's no images of blood or gore, but the songs are full of them (i.e. divorce, beheaded, and die; divorce, beheaded, survive for Henry VIII's wives).
Horrible Science series has books like Chemical Chaos or Blood, Bones, or Body Parts. Ironically, I didn't find them as gross as the Grossology series, which seems to be full of bodily fluids.
