There are three styles of play Open, Narrative, and Matched Play with Matched play using points. Open is the "default" by dint of the fact that you only need buy models to play. The core rules are free as are the rules for all the units, bar the point values, via the app or downloading them from the models store page.

While the Core Book you can buy has rules for Matched play you need either the Generals Hand Book (GHB) for the Pitched Battle Profile which have point costs and unit sizes etc. The GHB has profiles for all units in the game and a version will be released annually released annually to balance. This is why while you also get the Profiles in army Battle Tombs you're required to use the most recently published Profile during play which in most cases is going to be what's in the GHB. This guide has more details.

All of which basically means what you "need" really depends on what you want. For example I just got started by buying the Soul Wars box, which comes with a copy of the Core Book. I'm interested in building out an army of Nighthaunts so I'm going to be picking up Battle Tomb which, having just been released, has their current Pitched Battle Profiles. I won't need the GHB as the Battle Tomb and the Core Book have everything I need to play a Nighthaunt army in match play.

I'm sure I've missed thing out but basically you can pick up some models, download the core rules, and you can start playing. If you want to play matched play you'll need to buy at least one book.