I haven't done this much research since grad school! For a 2000 word manuscript, I've got at least 25 books out of my local library. Actually, I've got almost 50, but the rest are from the Children's room and those aren't part of the research.
The hardest part right now is deciding which theory of the first Americans is the one I want to use for the book. There are at least three viable options for when the continent was first populated. Most of my research has been trying to figure out which is the most viable (and of course interesting to third graders). Columbus and the Vikings are well documented so there's not much deviation - I just need to figure out which angle to write from.
I'm having a blast!