Certainly, the biggest one is presented at the end of first season, on what Candance (Ambyr Childers) is still alive. With this change, everything that involves the character in the series implies a major change in the event. There are also characters that do not appear, others that were invented and identities that are exchanged only in the production of Netflix.
See the main changes in the list below:
False identities, characters removed and invented
I don't know if it really is as easy and common to change identities when moving to Los Angeles as the plot of the series presents. For Kepnes, this feat is not so common and in his book, there is no one with false names (only artistic). Joe (Penn Badgley) moves to Los Angeles with his real name, and ironically, it is when he does accounts on social networks like himself, as it is the only way to get to live a life in the city - including getting a job. So the real Will (Robin Lord Taylor), which Joe holds in order to steal his life, never existed in the original work. It is worth mentioning that Jasper (Steven W. Bailey), to whom Will owes, is also a novelty of production.
Obviously, because Candance is actually dead in the book, she doesn't change her identity either. But the name she chooses in the series is a reference to an important character removed from the plot, Amy Adam is Joe's girlfriend, whom he meets at the New York bookstore where he worked, right after killing Beck (Elizabeth Lail). She hits him, assaulting the bookstore and that is why he moves, to take revenge on the girl. Her new life in Los Angeles is based on where she had gone and she is also behind the real reason for Henderson's death (Chris D'Elia), as he believed the comedian and Amy were dating.
Finally, friends of Love (Victoria Pedretti) and Ellie (Jenna Ortega), Delilah's sister (Carmela Zumbado) do not exist in the book. In reality, she is not a manager, but Harvey, a man who did not deserve a mention in the series. Delilah is just an upstairs resident. Ellie may have been the invention of the new plot, but it ends up falling into place. The girl is Joe's door to social media and opens up to a discussion of abuse, serving as the excuse for Joe to kill Henderson.
A different family
Joe arrives in Los Angeles going to work at Anavrin, where he meets Love and Forty (James Scully) that still work there. In the book, he starts working at Cal's bookstore (the one who works at the market in the series). By the way, the market really belongs to Love's parents, but it's called Pantry. The brothers do help in the market, but they don't actually work there. Forty still has his dream of working with films and Love is always helping his brother. She meets Joe at a casting audition she is doing for their new film, from which Joe appears thinking Amy will be there.
Parents are also quite different in the series. They are loving and even included that love in market sales, instructing cashiers to say "Pantry sends your love" with every purchase. Love refers to them as the "most passionate couple in the world", so they never had betrayal and they never abused their children. In reality, they pamper themselves into adulthood and they use the money as they please. Forty keeps asking for financing for a producer uncle, never for his father, because he wants to grow in the right way in his work.
Forty in the book is not a naturalistic style like in the series, but he is still addicted and constantly gets into trouble for the family, when he disappears on purpose because he thinks they love Love more than he does. Love lives in anguish with this and Joe tries to kill her brother to end the suffering he causes her.
Milo over there different
The series demoted Milo (Andrew Creer) to a simple “friend with benefit”, when in the book he was a kind of adopted brother of Love and Forty. While the production introduced the character as an Australian who was best friends with Love's Ex-Husband - who by the way was not deaf and even though he had cancer, did not actually die from the disease, but rather drowned himself when he was diagnosed - , and was always trying to win it over.
In Caroline's work, he still wants to win her over, but they are very close, he was raised with his family and it was with whom she lost her virginity - in addition to a few escapes over the years. Super talented, he even manages to produce a film, a novel starring just him and Love that drives Joe crazy with jealousy.
Not so many bodies like that
The original book is called Hidden Bodies for a reason. Joe kills without batting an eye in this work, collecting bodies while trying to get away from the mug he left at Peach's crime scene in the previous plot and some evidence is found about Beck's murder.
The series slightly decreased the number of deaths caused by the character. In addition to Henderson, Joe also kills Delilah after a night together and she discovers the tools he used to kill Henderson, in addition to police officer Robin Fincher (who in the series is portrayed as David and has an affair with Delilah), who is an aspiring actor and for chasing people to try an opportunity he finds Joe in plain clothes and ends up crossing his path until he almost finds out about his crimes. He is murdered in a plan devised by the protagonist during a cable trip made with Love's family.
In addition to these, there is the aforementioned attempt to murder Forty, when he finds him drugged and tries to kill him with pills, leaving the body in the desert. However, in a hurry, he doesn't check if the job was done and he survives, returning, threatening Joe and forcing him to write the scripts for him. It is worth mentioning that Forty pretends a partnership where Joe writes his ideals of script, turning it into something good, and ends up selling without giving him credit. No, they don't even write about Beck's book, that was an invention of Netflix's plot.
On the other hand, there is no mention of Joe killing his father in the book. Not even from his mother's cases and father's abuse. All we know is that she used to leave her son in the market for hours to date.
Joe + Love and their different relationship
The relationship between Joe and Love is portrayed quite differently in the book. They meet and spend a night at the hotel together, where they leave for the Quinn family's summer home and spend the holidays there for almost two months - Love, Joe, Milo and Forty-, until they leave for the recording. of the Love and Milo movie. Only after that do they actually return to Los Angeles, after Love grabs Joe in an attempt to rescue the mug he left behind, forcing him to confess everything he has done. Love forgives him, stating that she has already forgiven a lot of her brother and can forgive his sins and from that they start to live together for real
Interestingly, she never liked to cook as shown in the series, besides having an aversion to oral sex, a practice that Joe constantly complains about in her thoughts not to do and that chooses an exact moment to practice, serving as a proof of love for Joe. By the way, the panties he steals from her couldn't be stolen in the book, since she had a habit of throwing away her underwear after wearing it once.
She also never hid her relationship, she was always open about everything with her family. Of course, she was never a psychopath as shown in the series' twist. Love never killed anyone, that includes the nanny, whose story in the book is the murder of a dog, committed by Forty. He killed the family animal as a child, because he liked Love better.
The end of the season You it was very interesting, but the author delivers an even better one in her book. After confessing everything to Love, discovering that she is pregnant and accepting a life with her, Joe really changes and comes to forgive Amy when he meets her.
However, investigations were still going on and Dr. Nick, who was released for lack of evidence in Beck's murder, manages to discover that his suspected mysterious patient was Joe through social media. Among other evidences, the protagonist is arrested and thus finishes the work, stating that he will be released by the influential lawyers of Love's family. The plot ends leaving it open whether the other murders were discovered and what will happen to the character, questions that will be answered only in the new book.