As for God, his way is perfect: the word of the LORD is tried: he is a buckler to all those that trust in him.

Psalm 18:30

THE PERFECT SYMMETRY of the Christian Old Testament has been well documented by many scholars. My favorite is the overview given by J. Sidlow Baxter in his magnificent six-volume survey of the entire Bible called Explore the Book  in which he presented a detailed analysis of the main themes of each of the Sixty-Six Books in the context of the "big picture" of the whole Bible. His work shines with a rare brilliance, being thoroughly enlightened by his full appreciation of the Divine design of the Holy Bible, as is quite evident from the introduction to his book:

Our Bible consists of sixty-six component parts. These are divided into two distinctive major collections, the Old and New Testaments. But each of these two Testaments, the one consisting of thirty-nine books, the other of twenty-seven, is found to be arranged in certain clearly homogenous groups; and in this connection careful investigation reveals the presence of a marvelous Divine design running through the whole. ... This presence of plan and design does not only pertain to the Bible in this general sense; it runs through all the different book-groups considered separately; and the more we follow it through in detail, so the more wonderful it becomes, until all possibility of its being mere coincidence is eliminated by overwhelming abundance of evidence that this is indeed the word of the living God.

Baxter was not alone with this insight. W. Graham Scroggie held an identical view which he described in the introduction to his magnificent synthetic vision of the Bible as whole, The Unfolding Drama of Redemption  (1953):

In the Bible, as in biology, the whole is more than the aggregate of the parts. A living body is more than an assemblage of limbs; and the Bible is more than a collection of texts, paragraphs, chapters, or even books; it is a spiritual organism, in which each part is related to, and is dependent on, every other part, the whole being pervaded by spiritual life. ... It has a starting point, a track, a goal. The Temple of Truth is upreared from its foundation to its consummation by its glorious superstructure, in which are beauty of conception, unity of plan, harmony of parts, and growth towards completion.

This "presence of plan and design" that "runs through all the different book-groups considered separately" is seen in both the Old and New Testaments separately, as well as when they are taken together to form the Wheel. The table below shows the perfect symmetry of the Old Testament:

The details of theses canonical divisions are discussed more in the article called The Sevenfold Canon. When displayed on the Wheel, we see a coherent subdivision between Spokes 14 and 15 based on the Babylonian exile:

This is structure is discussed at length in Chapter 6 of the Bible Wheel book.