Alien monument mystery.
The strong point of 2001 as a novel is that it actually explains what the hell is going on in its movie counterpart, from the prehistoric man segments, right on through.
With the discovery fo the alien monument on the moon and the shock that engenders, through to the mission to Saturn to work out what is going with the alien monument makers, and the post-HAL phase with Bowman, leaving you with a far greater understanding and appreciation for what at times can seem a bit of a drug trip otherwise.
Definitely worth reading for that.
4 out of 5