Books that have left a mark
HPMOR (available for free here: http://www.hpmor.com/) is one of these books where you are scared of reaching the book's end. You can Think of it as an alternative universe story where the only difference is that Petunia did not merry Vernon Dursley, but ended up with an Oxford professor instead. Harry's difference in upbringing brings cascades of changes to the original storyline. I think I'll read this one again sometimes. Kudos to Eliezer, this is a great book.
Rama is thought-provoking with the right amount of details and science to stay relevant and not to spoil the mystery even though it was written almost fifty years ago.
Questioning and exploring the most efficient way to have a planet-like, self-sustaining environment in an interstellar spacecraft.
A lot of effort went into or provoked thought on the technological and scientific concepts needed for long-running interstellar or interplanetary travel. This book also leaves a lot of questions opened, but it is without a doubt one of my favorite books.
Even though the storyline of this book plays out in some distant future, one can not help but think that the original Brahmanian characters are plausible and could have displayed their power and status through advanced technology. The intriguing but flawlessly abstracted scientific concepts running in the background trough the whole story are the reason I enjoyed the book and that it resonates perfectly with Arthur C. Clarkes third law, that is, that any sufficiently advanced technology is indistinguishable from magic.