Maybe I shouldn’t be saying this on the official forums, but then again maybe this is a testament to why a Book 4 might be good. I don’t own a legal copy of the MB, but you’re right, it’s very useful. So useful in fact, that I went to my library to print and bind pages 14-104 with a custom cover.
Honestly even just an official PDF release of this old material would be worth it, and I’d happily buy it to pay recompense.
That said, if a full official printed Book 4 were released I’d want to see some major changes too. The Monster Burner is great as is, but it’s also a bit of a mess. And although I love the guidance in there on using the ‘language of traits’ to show how monsters differ from men, it also means that the more a monster deviates from that ‘human template’ the more bloated it gets with niche traits.
I’m also not convinced that the traits pricing system is properly balanced. ‘Affinity for …’ traits seems like a perfect example: 4 trait points to get +1D to a thing, when a call-on would be clearly superior for half the price. And +1D can just as easily (and much cheaper) be added with an appropriate Wise as a FoRK.
I’m honestly not sure what the fix to all this is though.
I can’t speak for Luke, obviously, but I do faintly recall a QnA he did on reddit a while back where someone asked him about the monster burner and he said he was never quite happy with it.