Paolo Giordano's book was so well-named that I borrowed his title for this post. I won't say this as eloquently as he does, but his imagery is so compelling. Mattia and Alice are prime numbers--unique and alone. They may be twin primes--two prime numbers separated only by an even number, like 41 and 43, but even twin primes cannot every get too close to each other.
Mattia is, in fact, a twin; a tragic twin, for his sister disappeared when they were children and has never been found. Alice has her own tragic story, although the causes of her eccentricities are not nearly so tangible. Remarkably, Alice and Mattia find each other...and their friendship, however tenuous, is formed.
It is a beautifully written book; the language alone makes it worth reading. Giordano's use of metaphor is compelling and beautiful; one of the best books I have read this year.