The Skylark of Space E.E. Smith Amazing Stories 1928.
"Not very much. Our figures show that with this four-hundred-pound bar"—pointing to the copper cylinder in the exact center of the inner sphere—"we could develop not only the velocity of light, but an acceleration equal to that velocity, were it not for the increase in mass at high velocities, as shown by Einstein and others. We can't go very fast near the earth, of course, as the friction of the air would melt the whole works in a few minutes. Until we get out of the atmosphere our speed will be limited by the ability of steel to withstand melting by the friction of the air to somewhere in the neighborhood of four or five thousand miles per hour, but out in space we can develop any speed we wish, up to that of light as a limit." Blockquote
This is from the original 1928 version, not later revisions. It is is the first reference to Einstein I found while skimming through it.
"About three hundred and fifty million miles," he stated. "Clear out of our solar system already, and from the distance covered he must have had a constant acceleration so as to approximate the velocity of light, and he is still going with full...."
"But nothing can possibly go that fast, Mart, it's impossible. How about Einstein's theory?"
"That is a theory, this measurement of distance is a fact, as you know from our tests."
"That's right. Another good theory gone to pot. But how do you account for his distance? D'you suppose he's lost control?"
The quotes are not working right.
But The Skylark of Space E.E. Smith Amazing Stories 1928 has two references to Einstein's theories (of Relativity) and there is no point in digging up later stories.
The "Black star" in a later chapter MIGHT be simply a burned out and dead star or it MIGHT be an early version of a neutron star or a black hole.