I'll go for three well written, radical metaphysical works of the 20th century:

Popper's Argument for Inderterminism is to my knowledge the only source of a workable metaphysics for modern physics. Very few philosophers have dealt with this issue, surprisingly.

Jacob's The Possible & The Actual attempts to the same for modern biology. It is short but very rich in ideas, clear, and to me, illuminating.

I would add Collingwood's Essay on Metaphysics, for its radically simple and effective way to conceptualize metaphysics.

Pretty sure the OP didn’t stipulate ‘philosophy books’ in particular or I would have answered differently.

For ‘must reads’ I have a fairly limited span in philosophical works but I would say:

- Critique of Pure Reason, because it is a challenge to read and requires concentration and study to get to grips with, as well as being one of the most important philosophical work ever produced.

- The Republic, because it is a great insight in ancient Greece and the origins of western philosophy.

- Being and Time, because it is awful yet a completely different and evasive style of writing that offers some nice points but ultimately outlines the border between meaningless drivel and brilliance.

- Something by Nietzsche, but do not start with Thus Spake … as he, like Heidegger, introduces a different style of writing to the world of philosophy.

- Philosophical Investigations, because it tackles important issues about language and language use, as well as being an amalgam of of ideas/thoughts.

I think these five give a pretty nice map of philosophy in terms of approach and style.

I still think Critique of Pure Reason is an essential read for anyone serious about philosophy. Second in line would probably be Being and Time. Not that I am a fan of Heidegger exactly, I just think it is important to see how obtuse philosophical texts can get and where, more or less, things went a bit off the rails.

, : Already mentioned

The very hungry caterpillar. — Banno

A tale of greed, of adversity, of toil, of satisfaction; of the fundamental fact that all things must pass; of overcoming oneself to achieve transformation.

Some are as great; there is none greater.

I would add Collingwood's Essay on Metaphysics, for its radically simple and effective way to conceptualize metaphysics. — Olivier5

I agree. I struggled for a long time with the idea of metaphysics. I wasn't sure what it is, but I knew what I want it to be. Collingwood helped me put words to that.

I struggled for a long time with the idea of metaphysics. — T Clark

Weren't you the one who introduced this great book here? Including to me. Tx for it. It did help me get more relaxed about metaphysics, more understanding of its importance, and able to reason at that level with more ease.

Popper's piece on indeterminism is a book I read out of sheer need for survival, at 20. I was finding the general consensus around determinism suffocating and toxic, not to mention scientifically outdated and not practical. For me, reading Popper was like drinking from a fresh and pure spring after a long walk in the desert. But even Popper, while writing page after page of excellent and lucid metaphysics, was under the (false) impression that he was staying away from metaphysics. Such was the negative connotation of this word even in his prodigious, original mind.

Weren't you the one who introduced this great book here? — Olivier5

It was Tim Wood who showed it to me.

It was Tim Wood who showed it to me. — T Clark

Bless be he.

Accurate list of books. Nevertheless, I think it is made for a West minded point of view. You shall not forget Asian books and culture, I think it would amaze you. I highlight what @T Clark wrote about Tao. I completely feel that is one of the most important books about thought and knowledge ever written. Give it a try.
I also recommend you these books:

• "The Haiku of the four seasons" by Matsuo Bashoo.
• "The story of Genji" by Murasaki Shikibu (紫 式部)
• "The Temple of the Golden Pavilion" by Yukio Mishima.
• "Snow Country" by Yasunari Kawabata.

I would also add 1984 by George Orwell, I think it would be valuable for society as a whole to read it (even though many people would choose to lie to themselves and ignore the book´s meaning...it´s only valuable for those who truly want to listen to the message).

Here are a few books that I would recommend others read, to better appreciate the techno-optimist perspective:

• "Where Wizards Stay Up Late" - by Katie Hafner
• "Virtually Sacred" - by Robert M Geraci
• "The Code Book" - by Simon Singh
• "Neuromancer* - by William Gibson
• "American Cosmic" - by D. W. Pasulka
• "TechGnosis" - by Erik Davis
• "Inner Sound" - by Jonathan Weinel

Generally speaking, I expect that most readers will have a positive reaction to the content and ideas found within the books recommended above. As the techno-optimist paradigm, is a "glass half-full" worldview.

As awful as a list of Marvel movies novelisations.

Must also add my voice to "must reads". It smacks of totalitarianism, no? Maybe that's too harsh. Similar to must watch this movie, or this youtube channel, or this top 10 best songs of all time; etc. But admittedly, I've read a few of the books suggested, and do not take exception to "The Gulag Archipelago" being a must read for high school students. When I went to school, way back, I appreciated all the books that were required reading in English class; and always remember fondly "The Mayor of Casterbridge" by Thomas Hardy (a great writer). Made an impression. So did Shakespeare's "MacBeth". Maybe better not "must" anything ... better ... suggest.

These merely reflect the respective impact on me, which of course is the only thing I can say.

Politics:

Understanding Power by Noam Chomsky

The Great War for Civilization by Robert Fisk

Never Let a Serious Crisis Go to Waste by Philip Mirowski

Philosophy:

A Treatise Concerning Eternal and Immutable Morality by Ralph Cudworth

An Essay Concerning Human Understanding by John Locke

The World as Will and Representation by Arthur Schopenhauer

Novels:

Novel Explosives by Jim Gauer

Grotesque by Natsuo Kirino

Ubik by Philip K. Dick

This forces me to leave our portions of books which I would otherwise recommend, such as Hume's Skepticism with Regard to the Senses which is a chapter, or Richard Burthogge's An Essay Upon Reason and the Nature of Spirits which I did not read in complete form, or indeed Kant's Solution of The Cosmological Idea of Totality in the Derivation of World Events from Their Causes,Possibility of the Causality through Freedom... Eludiation of the Cosmological Idea of A Freedom... or even essays, such as Chomsky's What Can We Understand? which I consider the most important essay in epistemology/metaphysics.

Same thing with fiction, I have to leave our portions of books, such as the first half of Michael Cisco's Animal Money, or the short stories of Borges, etc.

And I'm sure I'm leaving out stuff that I would kick myself for forgetting. But it's kind of inevitable.

It's almost impossible to write such a list, but it's an interesting exercise.

Must also add my voice to "must reads". It smacks of totalitarianism, no? Maybe that's too harsh. — Ray Liikanen

Is it not what you read. It is how you read ... and whether you can actually (rather than merely passing your eyes over some words vacantly).

I can barely read btw :) No shame in that though. I do my best and hopefully slowly improve here and there.

- Critique of Pure Reason, because it is a challenge to read and requires concentration and study to get to grips with, as well as being one of the most important philosophical work ever produced. — I like sushi

I've got a handle on what Kant's Critique is all about, but here one must allow Kant the courtesy of explaining himself. I suggest reading the challenge that Kant presents to his reviewer, found in the Appendix to his "Prolegomena ...". The challenge is open to all thinkers, philosophical or otherwise. To come to grips with the Critique one has to take to account the problems of metaphysics, mentioned in the intro to the Critique, as being God, Freedom, and Immortality. If these problems are not kept in mind throughout one's reading of the Critique, one will get lost in the forest of Kant's rhetoric, and so many of his readers do indeed get lost, having lost sight of these problems The Critique of Pure Reason is so called because it is a critical examination of how far pure reason can go toward resolving these probllems. The challenge found in the Appendix is a challenge that has not been met by any of Kant's followers, or detractors, or any neutral agent. I've given it a shot however, here's a link to my answer to Kant (but this is only if you want to understand Kant more fully--there is an Appendix included that quotes Kant and David Hume). Hegel is also mentioned in the body of the work, and the work can be considered in the Kantian sense, a science of metaphysics):

https://philarchive.org/archive/LIIRTP