Book by Richard Dawkins published in 2006
A must-read classic. It greatly enhanced my mental model of evolution and of biology. The author is humble throughout, not in the dubiously humble sense—but in the respectful-to-science way. It is filled with perspective on the biology that we encounter every day (such as suggesting organs may have evolved as beneficial parasites and eventually merged with the body). Having read a fair amount of Origin of Species (which is fascinating to read as a primary source), I'd say The Selfish Gene is where anyone should start enhancing their understanding of evolutionary biology. He uses all the tools Darwin didn't have at their disposal: game theory, genetics, modern medicine, and many others. It is not dense by any means and scattered throughout the book are stories of animals being dicks, such as a parasite that secretes a hormone to cause its larvae home to never turn butterfly and become abnormally large. Or cuckoos, that lay their eggs in other birds' nests and have some type of irresistibly cute gape that birds of other species will give them food before their own (?!). So many of these questions you've likely had your entire life since learning about evolution have a satisfying answer buried in the book: why did females and males evolve, how did cells start replicating, ..