"私は母が泣くのを一度しか見たことがありませんがそれは本の終わりでした"

Simplified Meaning:

The speaker is saying that their mother is usually very strong and doesn't show her emotions often. But one time, when she finished reading a book, she cried. This shows how powerful stories can be. Sometimes, a book can touch our hearts in a way that real-life events might not. For example, imagine reading a story about a character who goes through many struggles and finally finds happiness. You might get so connected to the character that you feel their pain and joy deeply. Just like how people cry at the end of a sad movie or when a song reminds them of something important, a book can also make us feel intense emotions. Even if we are usually tough and don’t cry much, a good story can move us to tears. So, if you find that things in life don't usually make you emotional, but a good book does, it’s a reminder of the unique power of storytelling. It’s okay to let yourself get lost in a story and feel for the characters. This emotional response shows our deep connection to the stories we read and how they can reflect our own experiences and feelings.