When I read the news on the Mystery*File blog this morning, I hoped there was some mistake, but Donald Hamilton's death has been confirmed. This is truly bad news. I'd hoped that someday we'd see the rumored final Matt Helm novel in print, but I don't have much hope of that now.
Hamilton was a wonderful writer, as anyone who's ever read his books already knows. The Matt Helm series alone would be enough to establish him as an important contributor to the crime, mystery, and spy fields, but standalones like Line of Fire are, to me, even better. His Westerns are good, too, and several of them (most notably The Big Country) were filmed. The Matt Helm books were filmed as well, but not so notably.
Recently Hard Case Crime reprinted Night Walker, so at least that one's easily available. The Helm books turn up on eBay all the time, and they're cheap. If you've never read Hamilton's work, you should. It's top of the line, and I'm very sorry to hear he's gone.
Update: Steve Lewis of Mystery*File has confirmed that the unpublished Matt Helm novel does indeed exist. Read his comments here.