Introductions to Metaphysics
To learn some basics of metaphysics---in particular, about the very mysterious and persistent problem of universals---I followed the path from the left to the right of this picture. The "very short introduction" is often a good way to start from scratch, and then follow suggested recommendations from there. That's what I did, and I feel that all three books are excellent.
In contrast to physics textbooks, in philosophy, even in basic introductions, it matters a lot who is explaining it, as the authors may disagree on several aspects of the various accounts---their challenges or strengths---, or even in the classification of approaches toward a goal. So it is enriching to read at least two texts.
The three books are extremely clear. The very short introduction is just a very brief appetiser, meant to make the reader (or me) realise that I'm unclear about everything (what is a table? what is a circle? how does time pass? what is a cause?). Even more, that I'm unclear about very important things. The second book is austere in its words, clear in the messages, and gentle in the transitions. The third book is not so austere, very readable, and very helpful after the second book.
I find it all really fascinating. And at the same time puzzling that we (people in general, but particularly academics) are so shielded from such important matters. And that people have been thinking about these things for millennia, and that we use predicate agreement in every sentence we utter, and that yet we are not aware of the problem of universals, let alone be knowledgeable about its accounts and their challenges.
At a more proximate level, I was amazed and elated to discover that the tension between universality & undecidability appears, under a certain disguise, as an aspect of at least the realists' account of the problem of universals. I think that investigating universality & undecidability from the interdisciplinary perspective we are trying to take can be beneficial for physics, computation, and who knows, perhaps we can say something new about this very old and fascinating problem. This is all coming up in a conceptual paper on our framework for universality.