Erin Lowry on her book, “Broke Millennial Takes On Investing: A Beginner's Guide to Leveling Up Your Money.” She did the legwork on everything from opening accounts (what kind?), figuring out what to put in them (stocks, bonds, funds?), how to invest when you want to make an impact in the world and more. In Mailbag: How to handle a rental property that’s underwater, and financial tradeoffs (invest or pay down the mortgage?).