There is a popular saying among philosophers that all of Western philosophy is a footnote to Plato and Aristotle. Nothing supports this more effectively than Aristotle's Metaphysics. Though written in the fourth century B.C.E., Metaphysics grapples with the central problems in ontology that have held the attention of Western philosophers ever since. Moreso than Plato, Aristotle's language and views have provided the foundation of how we think about existence and the conundrums that it poses.
Richard Hope's translation of Metaphysics identifies "primary being" as the central object of Aristotle's study. The term is perhaps clumsy, but nonetheless apt in that Aristotle identifies and expounds on a number of "objects" that exist in some form or another, but which need to be distinguished from the most fundamental objects of reality. By making these distinctions, Aristotle is able to provide insight into what exists and what merely holds a relationship to what exists.
We can attribute to Aristotle many of the most basic notions with which we understand the world. At least we can find no earlier extensive exposition of these or similar ideas, with the exception of Plato; but even here, Plato is a myth-monger, overly reliant on metaphor in comparison to Aristotle. Indeed, the lack of comparable material during or before Aristotle is evidence that his genius was perhaps unrivaled in the Western tradition.
Metaphysics explores the relationships among objects and their attributes, attributes and other attributes, causation, change, materiality, space, number, ideas, unity, permanence, relationships, and a host of other ontological concepts. Many sections of the work are obscure, if not unintelligible, but other sections are lucid descriptions of the most basic structure of experience. Lurking just below the surface is Aristotle's logic. For example, the relationship of contraries appears again and again to explain the possibility of change, while Aristotle explicitly asserts that the law of non-contradiction is the most certain of all principles and that all demonstrations rest upon it. It is perhaps here that Aristotle's genius is most evident. While earlier philosophers employed the same reason as Aristotle, none did so consciously, and by doing so, Aristotle set a standard for intellectual discourse that has born valuable fruit through the millenia.
A careful reading (and re-reading) of Aristotle will certainly yield great rewards, particularly in understanding the details of his thinking, but even a casual reading pays dividends as a systematic and exhuastive meditation on the most fundamental concepts of existence focuses one's mind on the amazing gift that is human experience.