Sally Rooney's highly anticipated third book has been described by critics as her "best novel" and her "strongest writing thus far" - but also "a puzzle of a novel - brilliant and flawed".
Reviews of Beautiful World, Where Are You have just been published.
It is the follow-up to the Irish author's acclaimed Conversations With Friends and Normal People.
"For all its structural oddness, Beautiful World, Where Are You is Rooney's best novel," the Times said.
"Her prose is now free of moments of clumsiness - especially about metaphor - that marked Normal People and Conversations with Friends," the paper's critic James Marriott said.
"Her ideas are more fully, poetically developed.
"The emotional control and technical mastery of the book's final pages reveal her as a novelist who will soon be able to do more or less as she likes - if she's not there already."
The novel, which is published on Tuesday, has "a touching honesty and truthfulness... along with a quiet brilliance", according to the Financial Times' critic Diana Evans.
She wrote: "Overall, this is Rooney's strongest writing thus far, bathed in a soft white light - she does some wonderful things with light, and air - bearing (sometimes heavy) glimmers of James Salter and Proust in its dense, photographic mapping of everyday conscious experience, then spanning outwards to our small consequence in the vastness of the universe."
The plot follows an author called Alice, who deeply dislikes the scrutiny brought by her wild success, and her friend Eileen, who works for a literary journal.
The author has said she has used it as a way of working through the attention she has experienced herself.
"That is why I had to write this book," she told the New York Times, "because my life had become so dominated for a time by the success of my previous two."
The Telegraph's Susannah Goldsbrough agreed Rooney's new work included "some of her best writing", but had some reservations.
Large parts of the novel are told in the form of emails between Alice and Eileen, which Goldsbrough said slipped "between politics and dense reams of fact, so that they end up reading more like Wikipedia entries than fiction".
"It isn't that they are uninteresting - very few things that Rooney thinks or writes are," she wrote.
"The problem is that they bear only tenuous relation to the business of the novel.
"It's all the more frustrating because jammed between the emails is some of Rooney's most beautiful writing.
"She made her name with two books that treat the mechanics of first love with sincerity and painstaking, sometimes painful, attention to detail.
"With her third, she has brought that sensibility to bear on a long love, stretching unspoken down many years, with even richer results."
Writing in the Guardian, Anne Enright said fans of Rooney's previous work would "relish the ache and uncertainty of her characters' coming of age, her way with emotional difficulty and her brilliance in showing the barriers we put between ourselves and the love of others".
"The last third of Beautiful World, Where Are You, when the four characters meet and connect, is a tour de force," she said.
"The dialogue never falters and the prose burns up the page.
"It takes some time to get these people in the same room, however.
"And that movement towards intimacy is purposely delayed by Rooney's descriptive prose, which heats up slowly."
The publication comes a year after the acclaimed TV adaptation of 2018's Normal People.
Rooney's 2017 debut Conversations With Friends is currently being filmed with a cast including Joe Alwyn and Jemima Kirke.