I'm just going to recommend three books with "hollow" in the title.

Hollow Kingdom by Kira Jane Buxton

S.T., a domesticated crow, is a bird of simple pleasures: hanging out with his owner Big Jim, trading insults with Seattle's wild crows (i.e. "those idiots"), and enjoying the finest food humankind has to offer: Cheetos ®.

But when Big Jim's eyeball falls out of his head, S.T. starts to think something's not quite right. His tried-and-true remedies—from beak-delivered beer to the slobbering affection of Big Jim's loyal but dim-witted dog, Dennis—fail to cure Big Jim's debilitating malady. S.T. is left with no choice but to abandon his old life and venture out into a wild and frightening new world with his trusty steed Dennis, where he suddenly discovers that the neighbors are devouring one other. Local wildlife is abuzz with rumors of Seattle's dangerous new predators.

Humanity's extinction has seemingly arrived, and the only one determined to save it is a cowardly crow whose only knowledge of the world comes from TV.

What could possibly go wrong?

The Hollow Places by T Kingfisher

Pray they are hungry.

Kara finds the words in the mysterious bunker that she’s discovered behind a hole in the wall of her uncle’s house. Freshly divorced and living back at home, Kara now becomes obsessed with these cryptic words and starts exploring this peculiar area—only to discover that it holds portals to countless alternate realities. But these places are haunted by creatures that seem to hear thoughts…and the more one fears them, the stronger they become.

House of Hollow by Krystsl Sutherland

Iris Hollow and her two older sisters are unquestionably strange. Ever since they disappeared on a suburban street in Scotland as children only to return a month a later with no memory of what happened to them, odd, eerie occurrences seem to follow in their wake. And they're changing. First, their dark hair turned white. Then, their blue eyes slowly turned black. They have insatiable appetites yet never gain weight. People find them disturbingly intoxicating, unbearably beautiful, and inexplicably dangerous. But now, ten years later, seventeen-year-old Iris Hollow is doing all she can to fit in and graduate high school on time--something her two famously glamourous globe-trotting older sisters, Grey and Vivi, never managed to do. But when Grey goes missing without a trace, leaving behind bizarre clues as to what might have happened, Iris and Vivi are left to trace her last few days. They aren't the only ones looking for her though. As they brush against the supernatural they realize that the story they've been told about their past is unraveling and the world that returned them seemingly unharmed ten years ago, might just be calling them home.