This would normally be the post I would give you a couple of more states to study. However, there's some things that happen all month long that I want to spotlight first, so come back later in the month for that post.
February boasts President's Day and Black History month. Both of those require finding a good source for children's biographies. During my trek through the United States I've found some great sources that I will always head to first if they exist.
David Adler "A Picture Book Biography" series- He has written or co-written a book for almost every major historical figure in American history, and most people you would cover in Black History month. This is my go to biography books right now, it provides a fair amount of information for early elementary without overloading.
Rookie Reader biographies- These are great for preschoolers, or for a first/second grader doing a report on their own. Most of them are geared for later first grade readers, and it gives them the right amount of facts for an independent report. These also sometimes venture into athletes occasionally.
Jean Fritz biographies- These are mostly the biographies of "dead white guys," as my history teacher used to say. She specializes in American Revolution time period and a few Civil War books, but most of hers are the American Revolution. These are written at a 3rd grade reading level and are great early chapter books. When I was that age I nearly wore out my copy of "Shhh! We're Writing the Constitution." These books are great for a third or fourth grader writing a report. It provides a lot more meat to them and is written in a conversational style.
Do you have any biographers that you really enjoy? There's several other authors who have written some great books, but they are more of the one shot type of books.