It is rare for a sequel or the second in a series to be as good as the original story, the one that first presented the reader with this new world, these new characters. It is even rarer for it to be better. But The Raven is definitely more exciting, more of a page-turner, than the very good The Sentinel.
Jane Harper has been trapped in Greenland since the close of the first adventure when, unsurprisingly, virtually no one believes her story of what happened to the crews of the whaler and the anti-whaling ship that both sank following a ramming incident and an explosion. Unsurprisingly because the story she tells is replete with thousand-year-old zombie Vikings, and - worse still in the opinion of the media who decide what people shall and shall not believe - zombie polar bears and narwhals and whales.
Now though, the only other survivors, the elderly Captain of the whaler and his son, return to the island where it all began, intent on saving the world from the (alien) parasite that causes this living death, and they give Jane little choice but to accompany them.
I don't want to spoil the story. I will simply say that if you enjoyed The Sentinel, don't miss The Raven. And if you haven't read The Sentinel, then read it first: this book, The Raven, is not a stand-alone.