• In a survey of 500 project histories from real-world development efforts taken every year between 1977 to the publication of the book in 1987, 15% of software projects fail and 25% of software projects that take longer than 25 years fail.
• For the overwhelming majority of failed project, no single technological issue could explain the failure.
• Projects fail because of sociological reasons, not technological ones.
• Managers should prioritize managing work and not the act of doing the work itself.
• Tech companies engage in the business of human communication and not the act of researching fundamental breakthroughs; the breakthroughs have already been made. Tech companies just apply them.
• We tend to focus on technical work over the human side of work because it is easier.

The authors also state that the underlying thesis of the book is that:

The major problems of our work are not so much technological as sociological in nature.

And a good analogy they provide is:

If you find yourself concentrating on the technology rather than the sociology, you’re like the vaudeville character who loses his keys on a dark street and looks for them on the adjacent street because, as he explains, “The light is better there.”

Source: Peopleware: Productive Projects and Teams, 2nd Edition, Chapter 1