I wrote a post about the importance of humor in books, making your audience laugh, but tears can be just as moving as amusement. In fact, the books that make us cry can be as powerful as the books that make us laugh. Do you know how to make your readers cry?
A Series of Unfortunate Events
Tragedy happens in life, and books that contain tragedy will feel authentic if it's written well. Tragic events are the best way to make your readers cry, but you have to walk a very fine line.
Death is a common tragic event in books, and it can inspire tears...but it can also fall flat. It's all about the character who dies. You have to be part writer and part scientist to create the perfect destined-to-die character.
First, don't make them too obviously good. I read a book, once, where I was absolutely certain the sister was going to die because she was being painted as some sort of perfect saint. Then she died, and I didn't care because I knew all along she wasn't going to live. Don't do this. All people have good and bad in them. You want to create a character that readers really like (the better to pull the tears from them), but it has to be a believable character. Give them flaws and allow them to make mistakes.
Second, give the reader enough time to know the character. Put the character in several pivotal scenes and allow them to play a key role before you brutally kill them in a cruel attempt to make people cry (it's okay, you sadist; everything is allowed in fiction).
Heartbreak is one of the most common tragedies in books as well, but this is even trickier to write than death. First, you must make the main character and the audience fall for the love interest. You must make everyone trust this character implicitly. An heroic act can be a big help. If the love interest saves the main character from something terrible, for example, this can create a sense of trust. Maybe the love interest confesses to something wrong they did, and makes amends for it. Perhaps they stop someone else from doing something wrong. There are lots of ways to do it, but make the reader trust first.
That will make the subsequent betrayal hurt that much worse. Make the betrayal truly awful and write a wallowing scene where the main character is deep in it, and that should pull some tears out of your audience.
Find your own way to bring tragedy into your books, and make 'em cry. People like a good cry, every now and then, and crying over fictional characters is perfectly harmless.