Good resources for Tax Liens, Foreclosures and/or Mortgage Notes

That's a pretty broad range of subjects. You might want to start with some books that give you an overview until you figure out where you want to specialize and then drill in more. For mortgage notes Paper Profits by Josh Andrews or Note Investing Made Easier by Martin Saenz are good places to start. I've heard that Dave Van Horn's book is good as well but I haven't read it myself. I'm not sure about tax liens since that's not my area, but as John mentioned there are a ton of resources on this site.