All of Alice Oseman's books (OSEMANVERSE) in chronological order. All of these books are tied together, and there isn't really an order in which you should read them, but if you want you can follow the chronological one! Before reading, please look into the trigger warnings, as some of these books are really heavy. You can find trigger warnings on the book's page here, on StoryGraph. You can find the Heartstopper series on Webtoon or Tapas for free:
WebToon: https://www.webtoons.com/en/challenge/heartstopper/list?title_no=329660
Tapas: https://tapas.io/series/Heartstopper/info