"Les fourmis" of Bernard Werber was my first book, and one that got me thinking. Lots of philosophical ideas exposed in it, though just as well as a lot of mysticism.

But nevertheless, that was the first time someone got me to think about such things as the simulation argument, by making his characters observe a simulated universe, and then wonder if they themselves were simulated, or even the characters of a story ... which they were. I mean, I'd usually not really think about such an idea, I'd just dismiss it offhand without even a second thought or noticing that I had. This hooked me.