We started reading the magic tree house series after a trip to America where I think the books are more widely known. The stories follow Jack and Annie on their travels through time to various places in history in a magic tree house. The books are all 10 chapters in length and are all based around some form of historic event or time. For example, we've read about the pyramids, ancient Greece, Pompei and the wild west to name few.
As much as the books are fun fictional stories, the subject matter is factual and educational without children seeing it as such. Each story book does have an accompanying fact book although I can't comment on these as we haven't bought them.
They did run an English version of the books (which are American based) for the first few although eventually they went back to a common series. We're on book 22 now and I believe there's more than 40.
My 4 year old is too young to read them herself, although now she's learning to read at school she does read some of the words. They're probably better suited to 6+ as actual reading material. If like us though you're looking for a good bedtime series to get your teeth into then I can't recommend them enough.