Graham McNeill's Ultramarines Series / Necron books?

Any word of a seventh book coming out (I think "The Chapter's Due" is number 6)? Does the book seem to end in a way that there'd be a seventh? I'm on book five right now and I would like to know if I should start saving to buy Horus heresy books or just wait for a new Ultramarines book. I was planning on reading the HH books if the Ultramarines didn't have anything coming out.

Also - Are there any good Necron books out? Not necessarily based purely on Necrons, but with a decent amount of them in. "Bringer of Darkness" had Necrons for the last few chapters and the rest was how Pavonis fell into the state it's in. there were more Eldar than Necrons and the book's title reflects on a C'tan!