The movie Soylent Green was based on the novel Make Room! Make Room! by Harry Harrison.
A quick synopsis is this: Future New York is overpopulated as is the rest of the world. There is a massive green house effect causing too much heat. Martial law is in effect and electricity and food are severely restricted. The protaganist is a cop and is investigating a murder. This leads him into an ever more complex who dunnit. No one cares who is homeless or poor. Money, sex and power rules. If you want to, there are suicide stations so you can end your life. Because there are no farms, there is no meat. Most food comes from the sea in the form of modified plankton. This was originally made from soy and other plant protein but as farms went away another source was needed. Soylent green, yellow and red are made. The color changes every week or so. The book basically hints that soylent green contains some kind of animal protein but is never explicit about it.
The movie takes this idea one step further and has people as the main ingredient. The movie is only loosely based on the book. The book is sad and depressing, but a good read nonetheless.