There is a fusion list in game though it mostly give hints to one of the ingredient.
For Nel and the slime, it is mainly letting them work with your body or be with you.
For a Town of Magic wiki, that would definitely be a fine thing to be created.
I have the book but like... I know damn well I still have stuff that can be fused and it doesn't list them in the book :/
As for the wiki, I had actually found one that looked like it was very new with some limited info on some characters, but it seems to have been taken down for some reason. The site no longer exists lol.
So, the thing about the book, it only shows "new" stuff if you are holding an upgrade material like gold. But the way it does this is by doing ????? For the name then gold + ????? For the recipe under the name. I already know there are things I can upgrade, but those really do not help me make sure I got the ones I want if I don't know if I'm missing something. Hell, I wouldn't have thought to upgrade the loot finder if I hadn't seen a comment about it >_> and even then, I still wonder if there was a better option to use the silver on. Just like I am currently wondering what I should upgrade with the thing after gold.
Oh, for those, you can generally upgrade all of the equipment that boost your stats like the arcane ring that boosts damage as long as they look like they are made of metal. You should be able to collect the necessary metals to upgrade all of your equipment. The current upgrade path is x -> silver -> gold -> mithril. I recommend upgrading the loot finder and then the things that are important for damage. Maximizing resource collection and damage output speeds up the grind I find.