The Book of Changes is not one book, and separating its layers is the single most useful thing to know about it.
The oldest layer is the Zhou Yi: the sixty-four hexagrams with their judgments and line statements, dating to the Western Zhou period, roughly the early first millennium BCE. Tradition ascribes it to King Wen and the Duke of Zhou. Read on its own, without commentary, it is a divination record, terse, concrete, full of omens, sacrifices, captives, marriages and journeys, and with very little philosophy in it.
The philosophy is the second layer. The Ten Wings, commentaries traditionally ascribed to Confucius and now generally dated to the Warring States and early Han periods, are what turned a diviner's manual into a cosmological work. The Great Treatise among them supplies the account of yin and yang, of change as the fundamental fact, and of the sage who acts in accordance with the moment, ideas that became foundational for both Confucian and Taoist thought and that most readers now take to be what the book is about.
Excavated manuscripts have complicated the picture usefully. A silk text found at Mawangdui in 1973 preserves a different hexagram order and commentaries not in the received edition, which is direct evidence that the tradition was more various than the transmitted canon suggests.
In the West the book arrived largely through Richard Wilhelm's German translation and its English rendering, which carries an explicitly philosophical and psychological reading; Carl Jung's foreword to that edition introduced the notion of synchronicity as a way of accounting for the oracle without causation. That framing has shaped Western use ever since, and it is a twentieth-century framing rather than a Chinese one, which is worth keeping in view when the book is described as ancient wisdom.