[llambda]

How about His Master's Voice? (I've been told it's one of his more dense works, but I haven't yet gotten around to it.)

[joe_bleau]

Loved it. I've not read enough of Lem's output to compare it to his other work. A Lem SF novel that includes information theory works for me.

[StrawberryFrog]

That's near the heavy end.