American Scientist once published their list of the twelve greatest scientific mongraphs of the 20th century. I couldn't find the list on their site anymore but found enough mentions of it to cobble together an Amazon-linked booklist. For reasons not worth delving into, I felt like perusing that particular list again today.
Some of these books are outrageously expensive, some are out of print, some can only be bought used. Many are somewhat outdated now, but they're like LPs--they're worth collecting for their historical aura.
John von Neumann and Oskar Morgenstern: Theory of Games and Economic Behavior (1944)
Paul Dirac: The Principles of Quantum Mechanics (1930)
Albert Einstein: The Collected Papers of Albert Einstein (1930)
Benoit B. Mandelbrot: Fractals: Form, Chance, and Dimension (1977)
Linus Pauling: The Nature of the Chemical Bond and the Structure of Molecules and Crystals; An Introduction to Modern Structural Chemistry (1939)
Bertrand Russell and Alfred North Whitehead: Principia Mathematica 3 Vol. Set (1910-13)
Cyril Smith: Search For Structure (1981)
Norbert Weiner: Cybernetics (1948)
R. B. Woodward and Roald Hoffmann: Conservation of Orbital Symmetry (1970)
Albert Einstein: The Meaning of Relativity (1922)
Richard Feynman: QED (1985)
Donald Knuth: The Art of Computer Programming: Volumes 1-3 Boxed Set (1968)
Another more recent list is the 20 greatest engineering achievements of the 20th century:
1. Electrification
2. Automobile
3. Airplane
4. Water Supply and Distribution
5. Electronics
6. Radio and Television
7. Agricultural Mechanization
8. Computers
9. Telephone
10. Air Conditioning and Refrigeration
11. Highways
12. Spacecraft
13. Internet
14. Imaging
15. Household Appliances
16. Health Technologies
17. Petroleum and Petrochemical Technologies
18. Laser and Fiber Optics
19. Nuclear Technologies
20. High-performance Materials
That's a good list. Maybe I'd add credit cards. Make that airline mile credit cards.