If Goodreads offered the option to do half-stars, I'd give 3.5 to this book, splitting the difference between the solid 4 it deserves for concept and the 3 it deserves for execution.
The idea behind the book is intriguing. It's a deep exploration into loneliness, into the way the walls we build to protect ourselves can all too easily turn into cages. It's a thesis on whether whether human beings have - and should - overcome our more animal instincts. It's a love song to finding inner strength in unexpected places.
At its core, How to Be Human is a comparison of one woman in two relationships - Mary and Mark, and Mary and the fox. Rather deliberately, the human relationship is so twisted and disturbing that it makes the animal relationship seem normal and healthy by comparison.
It's a passionately introspective character study, and unfortunately it succumbs to one of my biggest pet peeves for stories of this type: the character in question is far too self-aware, throughout the book, of her own shortcomings, drives, and growth. The protagonist, Mary, often opines at length upon her own story, to the point where she blatantly points out metaphors between her feelings and the events happening around her. At the book's conclusion, she simply snaps out of an extended psychosis by realizing it was, in fact, a breakdown, and ends by being suddenly ready to return to a normal life.
Where the book lost the most points for me was in the way it treated Mary and Mark. Yes, by the end she finally called him out for what he was - a dangerous stalker - but she does so in a fit of delusion, convinced that she and the fox are building some sort of life together. It was far too little, far too late. Perhaps if Mark's disturbing, predatory behavior had been more of a slow reveal, making the reader wonder at first if Mary's hostility toward him was warranted, it would have felt more satisfying to have her snap and condemn him in the end. It also would have served as a nice mirror against her dependency on the fox, which does build slowly and beautifully from mere curiosity to all-out obsession. But from his very first appearance, Mark was a terrifying and emotionally abusive figure, which made Mary's eventual breakdown seem inevitable rather than surprising.
Overall, I found this book to be a bold examination of an interesting woman's self-imposed isolation, albeit one that occasionally gets a bit lost in the woods.