Welcome aboard! We're a small community here, but active. Please feel free to pepper us with questions.
It, uh, helps with community visibility.
The green-trimmed 'x Core' books are replacements for the red/tan-trimmed books. They're fundamentally the same, with some language changes, class touch-ups, and minor rule updates. The mapping from old books to replacement books is:
- Core Rulebook -> Player Core
- Advanced Players Guide -> Player Core 2
- Gamemastry Guide -> GM Core
- Monster Core -> Bestiary
It's worth keeping in mind that the contents of all of the rule books is available for free in multiple places online, most famously on the Archives of Nethys website, but also piecewise on pf2easy.com.