Via CQ, Mother Jones has a story about an effort to resurrect Haldeman's possibly missing notes from the conversation that produced the 18 1/2 minute gap. (This was a very early post-breakin conversation between Nixon and Haldeman; the disclosure of the gap in the tape, which odds are was caused a deliberate erasure by Nixon, was one of the huge hits Nixon took in the public relations portion of his demise). Truth be told, I wouldn't expect much, although I'm certainly interested. Remember that Nixon had maintained not only that he had no prior knowledge of the break-in, but that he had no knowledge of the cover-up until nine months later; the tape erasure, assuming Nixon did it, could have been motivated only by the cover-up of the cover-up.
In other words, there are three classes of things Nixon might have wanted to hide:
1. The cover-up of the cover-up: Nixon was involved in the cover up immediately after the break-in. We know probably all there is to know about that, although more fun details are always welcome.
2. Prior knowledge of the break-in: No one knows for a fact that Nixon had prior knowledge, and for that matter Haldeman always denied prior knowledge (the evidence is that he certainly knew about the general program at the Committee to Re-Elect, but IIRC there's no specific finding that he clearly knew about the break-in in advance).
3. Other stuff. There are all kinds of wacky conspiracy theories out there, none of which are of great interest to me, but what I do think is possible is that there may be additional operations of the Plumbers that have never come out -- after the break-in, a lot of people destroyed a lot of evidence (although they also failed to destroy some stuff), and it's by no means clear that the Plumbers ever decided to tell all. As with #1, new details won't change the story in any important ways, but might be way fun anyway.
For newcomers, I recommend Fred Emery's Watergate as the essential starting point on the subject, although I'd be glad to hear any more recent recommendations.