You've gotten good advice, but here's one more thing to consider. If you only do this once or twice, you are risking great, big info dumps in the middle of your novel. If there is any way to incorporate the information that would be carried in these chapters into the main story, that would probably be better. JKR was once asked if Harry and Ron were ever going to read "Hogwarts: A History" and she said, "Absolutely not." (paraphrase) Each time Hermione explained something to Harry and Ron, she also explained it to the readers, but instead of pulling the readers out of the story, it drew them further in.