A couple of weeks ago I wrote a story about Google in which I discovered in passing that the question "what is love?" was almost always among the top 10 queries, minute by minute, to the search engine. In future Google might do well to point the askers of this oldest question in the direction of Andrew Solomon's extraordinary book. In my experience of the past few days you don't so much read Far from the Tree as cohabit with it; its stories take up residence in your head and heart, messily unpack themselves and refuse to leave. Once there, as one, or a dozen, working answers to the most urgent of inquiries they prove hard to argue with.

The 976 pages began for Solomon 10 years ago as a kind of quest, and like all writer's quests it was, to start with, an effort on his part to understand himself as much as the world. The book seems a kind of affirmative sequel to the author's previous landmark volume, The Noonday Demon, published in 2001, which explored with poetic rigour the debilities of depression, in particular his own, into which he had fallen following the death of his mother, an act of planned suicide following a terminal cancer diagnosis.

Solomon, a magazine journalist based in New York, begins again inside his own head, with the impulses that made him become a writer – the sense of difference and dislocation wrought by severe dyslexia as a child, and by the understanding that he was gay in his teens; alienations that were mitigated by the indefatigable efforts of his parents to have him live comfortably from infancy in a world of words, and by his own troubled efforts to have his mother and father and others understand his sexuality. This imprisoning solipsism is quickly willed into something entirely different, however, when Solomon sets out on his search for those who make his own psychological anxieties and challenges, his difficulties of acceptance and filial frustration, seem something not only manageable but trivial.

This journey takes him to what he begins by imagining might be the outer edges of parental attachment. "The children I describe have conditions that are alien to their parents," he says of this stubborn and compendious inquiry, "they are deaf or dwarfs; they have Down's syndrome, autism, schizophrenia or multiple severe disabilities; they are prodigies; they are people who are conceived in rape or commit crimes; they are transgender."

Each of these groups is given a chapter to itself. And each chapter – like a series of discrete books – involves up to a dozen tales of how particular children have challenged their parents and the author with what they know of life and love. If that makes the book sound mawkish or exploitative, or a misery memoir on a grand scale, it never feels at all like that. Solomon never tries to draw explicit lessons from the families he talks to, and in defiance of his surname he continually stops short of judgment. Instead he details the often painful, occasionally triumphant, sometimes unbearable, always deeply human narratives with care and empathy, and from time to time illuminates them with the urgent politics and telling historical contexts in which they exist. Solomon interviewed, compulsively, more than 300 families for the book, and ended up, he says, with 40,000 pages of notes. It is odd to read something of this length that feels like a distillation, a piece of concentrated intelligence, but that is, nonetheless, its effect.

If Solomon has a thesis, it is contained in the book's opening line. "There is," he asserts, "no such thing as reproduction." Much as we might like to believe otherwise, his chapters make the unarguable case that our children are always importantly more different from us than they are alike. There is no normal in these pages, still less beyond them. "All offspring are startling to their parents," Solomon writes. He gestures towards counting the ways, and demands we do the same.

Given that this concerted act of looking and learning works by accumulation, it does the book a disservice to single out any particular child or family. You meet, in no special order, the irrepressible Harry Wieder, "among the most vital activists in the dwarf community", who is "gay, nearly deaf, often incontinent and the only child of Holocaust survivors"; Clinton Brown III, born with a rare disorder called diastrophic dysplasia, a "deluxe package of disability" that includes a lack of joints in hands or legs, and, in his case, a huge and courageous heart; a handful of the 30,000 children that are born as a result of rape in the US each year, and the mothers who learn to love them. Solomon looks for no hierarchy of suffering or difference, though his book makes a good antidote to "whiners" in whose company he includes himself. Neither does he always find love rewarded – among the families he would most like to have grown up in, he suggests, is that of Dylan Klebold, one of the killers at Columbine high school, whose parents, Tom and Sue, inhabit their "own private Oresteia" still in the house they always lived in, and have nevertheless found a way to love and forgive their son even as they live each day with the horror he perpetrated for no clear reason: "I know it would have been better for the world if Dylan had never been born," Sue Klebold says. "But I believe it would not have been better for me."

Far from the Tree is not a self-help manual, though it would be hard to read it and not count your blessings. In all of the multitude of families, no parental strategy, beyond dogged patience and tremendous hope, proves more viable than any other. The real tragedies seem to lie with those people who don't see a way to try. The mother whose elder daughter is a cheerleader, who gives away her younger daughter, a dwarf, "because [she] was never going to be a cheerleader in Westchester so she couldn't love her". Otherwise, "all parents are broken and full of error," Solomon writes at one point. "Being hurt by those you love is awful, but it is less awful if you know they mean to help." Whether gifted or slow at learning, helpless or disturbed, "pushing children can backfire; not pushing them can backfire too". As Leonard Bernstein's father, when asked why he had opposed his gifted son's career, replied: "How did I know he was going to be Leonard Bernstein?"

In some way the result is a travel book. About halfway through, Solomon mentions how many of his interviewees have found a useful metaphor in the short fable "Welcome to Holland" written by Emily Perl Kingsley in 1987, and since widely circulated among parents who are told that their newborn will not be the child they expected. "When you're going to have a baby it's like planning a fabulous vacation trip – to Italy," Kingsley, herself the mother of a son with Down's syndrome, wrote. "When the plane lands, however, the stewardess comes in and says, 'Welcome to Holland'… It's slower paced than Italy, less flashy than Italy. But after you catch your breath, you begin to notice that Holland has windmills… and Holland has tulips. Holland even has Rembrandts."

When Solomon himself returns from his epic travels in these lands he finds himself changed from the person he was, and, to his surprise, ready to become an unconventional father himself, first with a childless friend, then, through a surrogate, with his new husband: "I started this book to forgive my parents and ended it by becoming a parent," he writes, armed only with the only lesson, beyond humility, he has learned and conveyed with certainty, that "parenting is no sport for perfectionists". It seems a good place to start.