You can check Lean Six Sigma For Dummies by John Morgan, Martin Brenig-Jones.
Also, you can get green belt certification from Coursera (I just found this one out from Georgia Regent). Getting green belt is comparatively easy and do not require any pre-requisites. Also, later if you feel like pursuing a career as a PM or Continuous Improvement Manager, you can opt-in for Black Belt.
Hope this helps.