Since we’re talking about digital books. . .

. . . I would be remiss if I did not plug my current favorite science fiction novel, Blindsight.  It’s a nihilistic hard-scifi first contact story, dealing with the nature of consciousness, the Fermi Paradox, and biologically plausible space vampires.  It will melt your brains, and it’s available for free due to a tiff between the author and his publisher.