What book, for whatever reason, became important to you?

I’ve been going through my things and decluttering. Having always been a bookworn and loath to throw anything away I’ve had to do a very large purge of books. Some are impossible to let go of though.

One in particular, "A Spot of Bother" by Mark Haddon, will probably stay indefinity.
I’ve found many books that have made an impact but the ones that left the biggest marks and emotional impacts were the "right" books at the right time.

I was extremely depressed at one point in my life, not able to do much but lie in bed and stare at the ceiling with the curtains closed. Surrounded by books, for some reason the one I tried to read was this one. It took months, and at the end I felt like I’d lived in it. I’ll never read it again, because who knows what I’d think of it now. I don’t even know if it would be seen as objectively good. But it’s probably one of the books I’ve connected the most with for these reasons.

What book left a permanent mark on you? What story did you feel unusually connected to for whatever reason?
What book found you at the "right" time?