If you are using Dukeminier, honestly, the book itself is awesome (for a casebook). I use that with Gilbert to supplement, but usually my professor likes to pick out little things in the cases not covered in Gilbert, or other commercial outlines (which again, makes the casebook so good). E&E is not all that good, but it helps indeed, read it after you cover it in class and you will understand the lecture better!
Something else which may help you is Moynihan "Introduction to the Law of Real Property" which covers Future Interests thoroughly.