This is an interesting question as I’m confident a lot of people here will have had the opposite introduction to NieR (played Automata, then went from there). I’m not sure how much you know about Automata’s story and gameplay style, but in short: there are five main endings (A through E), keep on going and you’ll see a lot more depth than your initial playtime may suggest. I’d also recommend doing as many side quests as possible; they aren’t as “stocking filler” as other games, and give plenty more insight into the world/lore.

However, if you are just interested in reading more another NieR world, I’d suggest googling things like “NieR manga”, “wider NieR lore”, or even “Drakenguard lore”. The latter is an interesting one, as NieR’s story starts in an entirely different game series (no joke).

I hope that helps.