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.
This world is an untapped resource and they cannot see it. The Grand Library of Menaphos contains many texts that reference terrible atrocities of the past. Death on a scale beyond comprehension; a dark cataclysm that must have severed so many souls from their bodies.
Such magic is inefficient and messy. It tears life apart and leaves souls ripped and torn. It leaves the soul incomplete.
It does, however, mean there is that energy just waiting to be harnessed. Death on such a scale would mean that I might be able to achieve something I had hitherto thought impossible.
The arrival of [player name] can only mean that I succeeded.
I shall bring this world into a glorious new future.
They will thank me one day.