I had it in mind to review Mortiis' 'Secrets of My Kingdom', a book, which in fact did come packaged with one of his albums ('The Stargate'). Can this be added to the encylopaedia as a work in its own right? Or are such releases forbidden? In this case, it does seems indelibly linked with the music, and whilst I can clearly see the argument for not including every novel, poem, essay or even magazine column produced by an otherwise metal musican, could this example be considered? Or ought I to have read through the site rules more thoroughly?...

The example you brough up; no. That's not a stand-alone release, it only came very limited with The Stargate. That means, roughly said, that it was to be considered a 'gadget' or a 'limited packaging' for The Stargate.

I forgot to add: You can of course make a review of The Stargate including the book, with emphasis on the book. I don't see any problem with that as long as you also review the music and very important (in this case) how the music and the book connects.

This, for the reasons outlined. But even if the band published a book separately we wouldn't include it in the -wait for it- discography. That stuff's for the additional notes.