In her letter, she didn't say it directly, but if you read between the lines, you can tell she was unhappy.

read 是动词"阅读"的基本形式。 between 是介词"在...之间"。 the 是定冠词"这个/那个"。 lines 是名词 line 的复数形式，指"文字行"。 read between the lines 是固定短语，字面意思是"阅读字里行间"，实际指"领会言外之意"或"理解隐含的意思"。