Finding this text is a requirement for the Master quest cape. See Tips and Tricks to obtaining the Master Quest Cape for assistance obtaining the Master quest cape.
If lost or destroyed, a new copy can be obtained from the bookcase in your Player-Owned House made through Construction.
Below is the text you can read from within the book.
Interrogation of the prisoner goes poorly. Even using repeated mind-clouding magics and revealing the futility of his situation has not broken him. I may need to switch to more destructive methods, but I am loathe to capture another of the knights from the ill-fated expedition. If too many of them go missing at once then people will make connections.
This, more than anything, is what galls me about this world. We have to deal with 'heroes' constantly launching attacks against us for threatening their False-Using 'gods'. Half of them are mortals bloated with stolen power. The other half are alien entities, talking simulacra and worse that are also bloated with stolen power! How can they think any prayer or sacrifice draws the notice of any god? They make war while the gods are elsewhere, build them monuments they never visit and write reams of fawning scripture that many of these gods are too ignorant to be capable of reading.
The one thing Tarshak is good for is talking, and everything he has said has sickened me on one level or another. He has chased the V User across worlds without end, always failing due to his incompetence. And always he finds mortals bowing and scraping before 'gods' that would slaughter them all to get access to the Stone for a bare instant. He has seen people worshipping giant pigs, giant plants and giant rocks. He has seen them throw their own children into living flames. He has seen them engage in fights to the death with combatants in the billions. He has seen them welcome the obsidian hand of death around their throat with a smile. The sooner we massacre these deluded fools and the things they worship, the better. This is why I must break this knight and learn Saradomin's flaw. With that, and the method we have learned, we will be able to bring ruin to the widest-worshiped False User in this world.