Zarl Kruger's book appears to be a predominantly a record of the construction of the Tower of Granfordir, although there are references to other matters as well. It is written in Quenya. The text covers all the arrangements that were made to design and build the tower, including such details as the number of and type of workers and where the labour and material came from. It isn't entirely clear what the motive was for keeping such a record.

The description of the building matches what you know from having been there and there don't seem to be any references to parts that you didn't discover.

The practice of sacrifice featured heavily in the building process. Sacrifices were made before the building commenced and to mark the completion of each major stage. The mortar used to bind the stones was mixed with sacrificial blood. Zarl Kruger regarded this as a means of ensuring the ongoing arcane protection of the building and its contents.

Though Bael's Orb and Staff hide and protect Torlich from the ravages of time and tide, yet shall the very stones of Granfordir have the power to persist and endure just as solidly.

After a description of one sacrifice there is a passage....

Now that the Irtsercht is complete, my arm shall hold sway over this place for days without number. There will be such a power here as to turn the will of any man to my path.

After apparently visiting a local lord

The fool Grimeon thinks to oppose me. And all for the want of his pathetic hunting rights. With the Varnak's Eye I saw him plying the King with wine and pleading for his ridiculous justice. As if either of them could hope to stand against me. But for the time I do not wish to be disturbed and so I sent my power through the Eye and I turned the King's mind against Grimeon. He will have to find his petty sport elsewhere.

When describing the construction of the underground chambers.....

Bael prepared the Tsiechetres for the signs. It was carried in 3 days across Desus from Ronsin up the Forknin to Brashik, overland south to Ner Eminchar and up the Currh-rephol to Granfordir.

At the completion of the building....

The work is done and the time is approaching. We Five will walk at His right hand and all living things will know us and fear us.

Shortly afterwards....

Bael is destroyed and Gamlich is cast down. The work and preparation of the Five is ruined by the cursed Chalan and their foul Solar protector. Scant consolation that the vile pair also perished. The apprentice Tornsor brought the Star to Caro and there we learned that the tower at Torlich is lost to us and we must wait a 100 great years before Arak-ra can be attempted again. The parts must be hidden until then. West and south, east and in the beyond. My rod remains and it is fitting to conceal it here in another tower. When the time of rising is come, I shall return to claim it. Terror will walk the land at last, and its name shall be Gamlich.

Later....

The five signs are one Only Ror-lercham-ror turns the key The five made as one. Worship, Destroy, Summon, Despoil, Sacrifice The five shall be one.

Towards the end.....

The green stag will play his part well I think. It is almost as easy to read the hearts and minds of these fools, as it is to observe them with the Eye. Soon he shall come to defeat me and I shall begin the long sleep. When the change comes I will eat that tender, obliging heart for my future.

There is also a separate leaf in the back of the book

Let it be stated that the Demon Mzjkkl and the Demonist Zarl-Kruger have entered into a contract. For a period of time commencing immediately upon the signing of this contract by Mzjkkl and Zarl-Kruger and to terminate when the Demon Gamlich appears on the Prime Material plane, the Demon Mzjkkl undertakes to seek and destroy all living, dead or undead with the single exception of Zarl-Kruger himself who enter the room in which this contract is made during the time in which the contract is in force and Zarl-Kruger undertakes to supply Mzjkkl one ring of magical power for each decade of service by Mzjkkl under this contract and to be delivered to Mzjkkl on expiration of the time period of this contract.