What is the mood in Eclipse?

The third book in the Twilight Saga, Eclipse, builds on events immediately following those in New Moon, but it is much darker, much more intense, and much more nerve-wracking. One could describe the mood of this novel as one of apprehension and strife.
Although pending nuptials loom on the horizon for Bella and Edward, as well as the question of her promised vampiric transformation, the young couple’s wedding plans are overshadowed by the prospect of an all-out war with Victoria and her army of preternaturally strong newborn vampires. The Cullens and their allies joining forces with the werewolf clan only exacerbates another point of conflict: the love triangle that has existed among Bella, Edward, and Jacob. Bella’s choice of mate will not only have repercussions for her safety, it will also affect her future relationship with Jacob.
When not filling the reader with a sense of foreboding, the novel imparts a great deal of backstory, most notably about the Volturi and the history of the werewolves.