Let's not drag up this whole song and dance again.
To the actual subject,
Buy General's Handbook 2021. It has the core rules, and rules for more competitive/attempt to be balanced Matched play. It also has point changes.
Do NOT Buy the Flesh-Eater Courts Battletome. It's going to hopefully be replaced by a 3.0 battletome for them soon. Get your rules from alternate places like Battlescribe Roster Editor, or 1d4chan Tactics pages. Warscrolls for your army can be found online. I recommend that you don't waste your money on a soon to be outdated book.
Battlescribe will have your Faction rules, subfaction rules, relics and command traits.
So will 1d4chan pages, but they're more prone to edgy/shock/whateverbuzzword humor, just given the community they come from. But, as far as giving the information, their pages are solid.
https://1d4chan.org/wiki/Age_of_Sigmar/Tactics/Death/Flesh-Eater_Courts