Malign Sorcery and Forbidden Powers sets are not just rules but also Endless Spell models for all factions (
FP also includes a terrain feature for any faction).
You also don't want the core rule book either. For rules only its not that good. Sure they are in there, but there's a huge chunk of lore and art that you just don't need for gaming.
What you want is the Gaming Book
https://www.games-workshop.com/en-GB/Age-Of-Sigmar-Gaming-Book-EN-2019
Which also includes the Malign Sorcery rules and some of the
FAQ/Errarta updates to the core rules.
It's a much smaller more practical book for gaming with which focuses purely on the rules alone.
Add to that the Generals Handbook and the Forbidden Powers IF you want to use those endless spells. Same for Malign Sorcery. Both those sets are worth their costs for the models if you want the models.
So
Game Book
Battletome
Generals Handbook2020 - though note you can get the points changes for free off the Warscroll builder on the
GW website.
Optional extras for endless spell models - noting that if there is just 1 spell you want you might convert/find it second hand on ebay etc..
Malign Sorcery
Forbidden Powers