Obtaining this book, and the 4 others, is required for the Master Quest Cape. You must read this book before you will be able to obtain the next one. You do not need to be on Slayer assignment to obtain these journals.
Below is the text you can read from within the book.
I have met them again - perhaps for their first time, though certainly not for mine. How strange it is to see them again after all this time, how their image has been warped by the passage of years and the inevitable drift of memories from one death to the next.
I once imagined them as a mighty warrior, seven feet tall, with weapons that were wreathed in arcane fire. But I suppose I had always known that was but a fantasy.
To see them again now is almost disappointing. Even as they take my life, I find myself wishing that they were more than they are. Is it odd that I wish for my murderer to be more imposing?
At their hands, I have died countless times, and shall die countless times more. Who can say how many? I died as I did before. All things, it seems, are cyclical.