Goodreads has tons of groups and one of them is called Monthly Recommendations hosted by Trina from the booktube channel Between Chapters and Kayla from Kayla Rayne. Each month there is a topic for which you can give recommendations to, and the topic for this month is Friendship Stories. A lot of books are about friendship or the friendship is one of the core elements of a lot of books, but I tried to choose books that have something special about the friendship in them. Also I'm not going to include the most obvious choices like Harry Potter in this list but books I think not so many people necessarily have heard of or read.
First I want to recommend The Lies of Locke Lamora (and the whole Gentlemen Bastard Sequence for that matter). Locke Lamora is a bit like Robin Hood and the Gentlemen Bastards are his Merry Men. They steal from the rich but keep it all themselves and live carefree life. In the middle of a con someone interferes their business and things start to get dangerous on several levels. The book includes flashbacks showing how the group came together and became such a seamless team of criminals.
The second book I'm going to recommend is The Kite Runner by Khaled Hosseini. This follows an Afghan boy called Amir and his family's servant's son Hassan, who are best friends despite being from different social classes. They live in 1970s Kabul in Afghanistan, where one day Hassan gets in trouble and Amir doesn't help him because he is a coward. This drives a wedge between them, and sometime after that the Soviet Union attacks Kabul and Amir escapes with his father to the United States. His past keeps haunting him even when he has grown up until he gets a chance to make it all better.
The third book I want to recommend is City of Thieves by David Benioff. This is a story about two young men, reserved Lev and confident Kolja, during the siege of Leningrad in WWII. They meet at jail and in exchange for their freedom they are told to find a dozen eggs for a colonel's daughter's wedding cake in five days. Since food is scarce during the siege that is almost an impossible task, but they try nevertheless and a lot of close calls, stumbling and serious danger follows. Even in the most desperate situations they have each others back right til the end.