I think we have another great series along the line of the Shadow Children series by Haddix as well. The kids in this series are all adopted, there are 36 of them. The book starts out with a plane showing up at a gate at the airport with no one on it other than these 36 babies. We flash to 13 years later when 2 of the kids find each other by chance, or so they think. All the kids are moving to within certain areas all grouped around the same towns. The 2 boys who are our main characters are Jonah and Chip. Jonah's sister Katherine helps the boys with their sleuthing when they find out something is up with their whole adoption. Turns out these 36 kids are all from the past, rescued by time travelers from the future, trying to save children or people from horrible tragedies. Only, the rich have decided to adopt these babies, and they want famous babies, such as a king from the 15th century. It leaves off with these 3 and another one of the children, Alex, being sent through time back to the 15th century. They want to fix what is going on, and be able to come back to the home they've lived and grown up in for 13 years. And that is where the 2nd book will start off, back in the time period. I'm looking forward to reading it after I'm done with my Mark Twain list.
Next is Cicada Summer by Andrea Beaty.