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The Magic Treehouse and Merlin Mission books are a great introduction to chapter books because they have both girl and boy protagonists, a familiar plotline, magic (who doesn't like magic?!), and exploits of historical interest. The series is second-grade reading material at the outset but migrates to fourth grade difficulty towards the end. This gradual acceleration of difficulty within the comfort zone of beloved characters and plotline really helps children to not just love reading but to continue reading ever more difficult material.
But for me, the real thrill of these books is that it introduces children to reading non-fiction books for pleasure. Sure, you learn a little bit about a particular time and place in history by reading the fictional Jack & Annie adventure. But the real meat comes when you then read the non-fiction companion. There are only research guides for about half of the missions, but they're all gems. Jack and Annie are still there, as little heads appear in the margins with them providing definitions for new vocabulary or interesting extra tidbits of information. And every so often, you'll see some chicken-scratch where Jack has written a brief summary of what you've read, just like he does in his journal entries in the adventures.
Most of the research guides seem to be a bit more difficult to read than the fictional books, but this is largely because of all the scientific vocabulary and names of ancient people and cities. If your child can read the mission, they should be able to read the accompanying research guide with adult help for pronunciation of difficult words.
My son is six years old, finishing the first grade, and we have read all but the last of this series, starting with _Dinosaurs Before Dark_ and its accompanying research guide. I would have never dreamed that such a young child could be so excited about so many different periods in history or about reading actual non-fiction for pleasure. I will admit that, given a choice, he chooses the adventure stories first. But when they run out (we usually maximize 4-for-3 promotions in our purchases), he is very ready to start the next research guide that's available. The books have also correlated well with the history we've explored academically this year. Where the official history curriculum has been superficial because of the nature of its scope, the research guides on _Mummies_ and _Ancient Greece_ have provided ample extra detail. I plan to have him re-read relevant guides and adventures as he learns about additional periods in history next year.