The chapter that contains the "supreme secret of the O.T.O. IX degree" in Crowley's Book of Lies is passage number 69.

Here is the chapter:

THE WAY TO SUCCEED, AND THE WAY TO SUCK EGGS!

1. This is the Holy Hexagram.
2. Plunge from the height, O God, and interlock with Man!
3. Plunge from the height, O Man, and interlock with Beast!
4. The Red Triangle is the descending tongue of grace; the Blue Triangle is the ascending tongue of prayer.
5. This Interchange, the Double Gift of Tongues, the Word of Double Power - ABRAHADABRA - is the sign of the GREAT WORK, for the GREAT WORK is accomplished in Silence. And behold is not that Word equal to Cheth, that is Cancer, whose Sigil is Cancer?
6. This Work also eats up itself, accomplishes its own end, nourishes the worker, leaves no seed, is perfect in itself.
7. Little children, love one another!

Many have speculated on the secret chapter, but this is the one. This is the truth, and the whole truth.

From Crowley's Confessions:

"One of these chapters bothered me. I could not write it. I invoked Dionysus with particular fervour, but still without success. I went off in desperation to `change my luck', by doing something entirely contrary to my inclinations. In the midst of my disgust, the spirit came over me, and I scribbled the chapter down by the light of a farthing dip.. When I read it over, I was as discontented as before, but I stuck it into the book in a sort of anger at myself as a deliberate act of spite towards my readers. "Shortly after publication, the O.H.O. (Outer Head of the O.T.O.) came to me. (At that time I did not realise that there was anything in the O.T.O. beyond a convenient compendium of the more important truths of Free Masonry.) He said that since I was acquainted with the supreme secret of the Order, I must be allowed the IX degree and obligated in regard to it. I protested that I knew no such secret. He said `But you have printed it in the plainest language'. I said that I could not have done so because I did not know it. He went to the book- shelves; taking out a copy of THE BOOK OF LIES, he pointed to a passage in the despised chapter. It instantly flashed upon me. The entire symbolism not only of Free Masonry but of many other traditions blazed upon my spiritual vision. From that moment the O.T.O. assumed its proper importance in my mind. I understood that I held in my hands the key to the future progress of humanity...."