I would highly recommend reading Philip K Dick. His numerous novels typically try to answer the question of what is real and what is human. He was very prolificv, writing something like 40 novels and dozens of short stories. I find that his novels are generally better than the shorts. Since his output varies in quality (he wrote 12 books in 2 years at one point) I would recommend some of the following:
-Do Andoids Dream of Electric Sheep (Blade Runner is based on this)
-The 3 Stigmata of Palmer Edritch
-Now Wait for Last Year
-Martian Time Slip
-Dr. Bloodmoney
-A Scanner Darkly
-Flow My Tears the Policeman Said
-A Mazae of Death
-The Man in the High Castle
-Ubik
All of his novels are relatively short 200-225 pages, but be forewarned they do start off a little slow, and his prose is not always the most compelling. What is interesting though are his ideas and his view of human nature.
Happy reading!