Okay, so Everneath is a made-up word. That makes our job a little harder, but hang in there.
Let's start with what the word means in the book. The Everneath is a realm between our world and the next, populated by immortals known as the Everliving. It's fueled by the energy of human Forfeits, with the rules enforced by the mysterious (and kind of scary) Shades.
Knowing what the Everneath is turns out to be important for the book's plot, since our main character, Nikki, spends a century there and it totally messes her up. In the book, we see both the events leading up to her agreeing to go there, and the aftermath of this decision, when she's trying to get her life back. So, in short: no Everneath, no book.
On a more meta level, titling the book Everneath captures the main struggle that Nikki goes through in the book: Does she take the easy path to immortality, which means she'll have to prey on the souls of others, or does she accept her fate to get sucked back to the Tunnels, despite it causing her to eternally suffer? In other words, does Nikki choose easy gratification or the right thing? This is a choice that resonates with pretty much everyone ever.
The fact that Everneath contains the word ever up front is also important. Ever is a long time, you know? But the decisions Nikki makes (going with Cole as a Forfeit in the first place, and everything after it) have consequences for what her fate will be after that—and specifically, whether she gets a happily ever after, or a miserably ever after.
The importance of ever is also reflected in the tattoo Jack gets. It says Ever Yours, which he reveals to Nikki during a dream-visit from when he's taken her place in the Tunnels: "The symbols were artistic versions of ancient Sanskrit words. They stood for eternity and belonging. Ever Yours, just as Jack had said. There was no way my subconscious could have come up with that explanation on its own" (33.47). As much as the Everneath offers an eternity of sorts, so, too, does Jack and life above ground—it's just a kind of eternity that includes death.
With all this emphasis on the word ever in the book, as well as the fact that the existence of the Everneath is super important for the book, we think Everneath is a pretty darn good title for the thing.