I am currently editing a SUPERB book on the long ending of Mark's Gospel (Mark 16:9–20). The author, Nicholas Lunn, argues that the long ending was not a later addition to the Gospel but was the original ending written by Mark. In this he is going against the majority view, but I can say that his case is not simply reasonable — it is knock-down brilliant! He demonstrates that the case for the long ending being original is highly probable. (Seriously — I am as surprised as you may be.) I think that after the publication of this book anyone who still wants to argue for the exclusion of 16:9–20 from the Gospel has an uphill struggle.
I'd always believed in the genuiness of the long ending, but then I've been a King James Bible reader and Textus Receptus Defender much of my life.