Recommendations for hard sci-fi novels on time travel include "Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction" by Paul Nahin, which explores both the genre and its philosophical implications. "The Proteus Operation" by James P. Hogan is highlighted as a standout read in this category. The "Ender" series by Orson Scott Card is noted for its accurate portrayal of relativistic time dilation, although it lacks time travel to the past and faster-than-light travel. While Card's work is praised for its scientific rigor, some elements, particularly in "Xenocide," are criticized for relying on implausible technological leaps. Overall, the discussion emphasizes the intersection of science fiction with philosophical themes in time travel narratives.