Right now we're in the middle of
Foundation, by Asimov. Which is a comparatively light/quick read. It's got a number of the amusing foibles of Golden Age SF (all the smoking, for example, and the near-total lack of female characters), but its large influence on the genre is clear. 40k getting the tech priests from here seems obvious, as well as the sheer scale and concept of the galactic imperium. I feel like
A Canticle for Leibowitz does the time skip thing better, but it's also later and I imagine Miller probably drew some inspiration from Asimov's example and improved on that element. It's interesting that they're both assembled from short stories originally published in serial form.