Years and years ago I gave a set of books, The Collected works of Theodore Sturgeon, away.
This was a series published by his estate, not widely available.
I just got an email today from the man I gave them to telling me that not only had he read all 8 of the series that I gave him but that he’d found the last 3 volumes that hadn’t been published yet when I gave him my set.
That is one of the coolest things to ever happen, no cost, great joy.