One of the more interesting tragedies of Superman's existence during the Golden Age was the fact that he really didn't know his birth parents at all compared to the Kents. But that changed amazingly in the Silver Age, as Jor-El became a virtual guest character in the magazines edited by Mort Weisinger.
And when he couldn't appear in person, he had an amazing habit of sending objects to his son over the years, well after his own death. For example, Krypto, Superboy's dog:
But that was far from the only time that Superboy received a communication from his father. In Adventure #232, an entire city block of Krypton lands (improbably) within walking distance of Smallville. Even more unlikely, it includes Kal-El's childhood home, where he discovers a plea from his father:
He easily handles the first two tasks, but is unable to locate the books that his father wanted him to read. So upset is he, that:
It turns out that the Kryptonians were so advanced that they had their books in movie form. One of them turns out suspiciously to be like Moby Dick, while the others sound even more sleep-inducing.
Apparently these missives from the past were popular with the readers, and so in Adventure #240, we got another one:
Of course as we know, the end of Krypton came unexpectedly early, and so it had taken the super-teacher all that time to locate Kal.
The letters from home continued into Superman's adulthood. In Superman #113, he discovers tapes from his father:
The tapes revealed that Jor-El had been the first Superman.
In Action #314, Aquaman discovers another set of tapes from Jor-El to his son, at the bottom of the ocean:
I talked about that story here.
Those are the ones I know; anybody aware of any others?