Other top columnists on this site have already brought your attention to Steven Pinker's recently published Enlightenment Now, which Bill Gates calls his "favorite book of all time." I read it last week (while on vacation), and I think I know why Gates praises it so highly.
(Note: The observations below about Gates are based on statements he's made in the past; his many book recommendations from previous years; his books, letters, and blog posts; and my own experience interviewing him back when he was still the CEO at Microsoft.)
1. There Is Real Science Behind Optimism
While there are thousands of books published every year about "positive thinking" and "optimism," the genre suffers from magical thinking. While such books often have a pseudo-scientific patina (usually backed by a self-serving "survey"), they're rife with unprovable, superstitious statements like "everything happens for a reason."
Enlightenment Now, by contrast, provides ample statistical evidence, drawn from multiple peer-reviewed studies, that science and humanism are measurably making the world a better place by alleviating human suffering and increasing human happiness.
As such, the book proves that much of the handwringing and doom-saying promulgated in the popular press, academia, and politics can't be justified on the facts. Far from putting the world at risk, science and humanism have been successful where religion has failed utterly.
Gates is known for his optimistic viewpoint toward life in general and technology in particular. Enlightenment Now provides a fact-based foundation for his long-held beliefs.
2. There Are Only Two True Existential Threats
In addition to providing a scientific basis for optimism, Enlightenment Now spends many pages ridiculing the widespread notion that everything in the world is going to hell in the proverbial hand basket. Pinker accomplishes this not through rhetoric or opinion, but by showing clearly that the facts don't support the paranoia.
Specifically, the fear of terrorism is vastly overblown (especially in the U.S.), the fear of genetically modified crops and vaccination is scientifically absurd, the threat from asteroid strikes and overpopulation is minimal, and A.I. taking over the world is logically absurd.
Pinker explains there are only two existential (i.e., end-of-the-world-as-we-know-it) threats: 1) climate change and 2) nuclear war, both of which would and could result in the death of billions and immense human suffering, while rendering entire geographical regions uninhabitable.
Gates's most recent public letter (issued from the Gates Foundation) contains a huge broadside against Trump, who has 1) withdrawn from the Paris Accords and 2) threatened preemptive nuclear war. Enlightenment Now buttresses Gates's criticism of Trump and Trumpism, both of which are directly opposed to Gates's humanitarian agenda.
3. Individuals Can and Do Make a Difference
Perhaps the most startling facts in Enlightenment Now are the (conservative) estimates of the positive effect specific technological innovators have had upon humanity's overall well-being and happiness. Karl Landsteiner's (who?) discovery of blood types in 1901, for example, has saved more than a billion lives.
Pinker cites a list of 100 scientists who, in total, have saved five billion lives, not just through medical advances but also through seemingly minor inventions (like the highway guardrail) that have conservatively prevented hundreds of millions of painful and life-threatening accidents.
Enlightenment Now also highlights the positive effects of technology upon human happiness. In particular, Pinker celebrates the smartphone, which, far from creating angst and disconnection, is actually helping to create more wealth and easier access to entertainment, both of which result in an overall increase in happiness.
Gates, of course, is an individual who has both alleviated human suffering and created wealth through technology. The Gates Foundation has saved millions of lives through disease prevention, and Gates played a major role in the birth of the PC, which democratized computing power.
In short, Enlightenment Now is Bill Gates's "favorite book of all time" because it's both a manifesto of ideas that Gates himself has espoused through the years, as well as a paean to individuals, like Gates, who have committed their time and money to changing the world for the better.