I think so much of what makes a book memorable or important are the circumstances of your life when you read it. I was a bit depressed and frequently bothered by suicidal thoughts as a student and so I was drawn to books which matched my mood. At the time I was completely unable to seek out any support or confide in anyone.
The Handmaid's Tale (Margaret Atwood) left a real impression on me that one can be strong for oneself and that perseverance in the face of despair can pay off, even that hope is worth hanging on to for its own sake. Everyone talks about the feminist message/warning but to me it will always be a book about hope.
Around the same period in my life I read Appointment in Samarra (John O' Hara) - which is very of its time (1930s), pretty depressing / tragic, and I'm not sure I would even recommend it - nevertheless there was a message for me personally about the futility of suicide and that one's perspective on how awful things are can be skewed.
This are probably the two most impactful books for me personally because of when I read them and my headspace at that time, not necessarily because they are my favourite books or the books I would most recommend (though I would wholeheartedly recommend the Handmaid's Tale).
I don't consider myself to be well read but I think Lord of the Rings is a great piece of literature which confers a real sense of meaning, and again a major theme is perseverance against the odds and holding on to hope, so perhaps that's just something resonant for me.