Any book that has a couple which breaks up in a way that makes you cry, make you sad or angry that this is happening. The couple has to be together before the break-up so it can't be two person who are on the way to being a couple and suddenly stops seeing each other before they become one or a couple that used to be together where one person in this couple wants them to be back together but is rejected. It also has to be a real break-up where the couple stops seeing each other after, not a break/fight between the couple where we can't determine if the relationship has ended or a fight that almost became a break-up but isn't.
If it's a serie that is following this couple romance, the break-up has to be in the book you are putting in this list, not in the previous book of the serie or the next one.