The odyssey because it's the basis function for most of western thought (wily male human outsmarts gods, witches, and suitors on a long journey home). The best implementation of the monomyth, imho.

Moby Dick because it's not just a book about whaling, it's a book about world philosophy with crazy tangents.

Fire Upon the Deep because it is the best representation of post-singularity technological implications. Was a big inspiration for me going into machine learning and biology.