GRAY MATTER Neurons make new connections during learning.Credit...Illustration from istockphoto.com I LOVE reading history, and the shelves in my living room are lined with fat, fact-filled books. There’s “The Hemingses of Monticello,” about the family of Thomas Jefferson’s slave mistress; there’s “House of Cards,” about the fall of Bear Stearns; there’s “Titan,” about John D. Rockefeller Sr. The