Beyond the two core rule books and the game mastery guide, what books would you guys suggest investing in next? just for general expansion of either the whole experience or adding something the base game is missing.
The Advanced Players Guide I believe is effectively the 3rd core rulebook in pathfinder. It is the book that made pathfinder what it is and not just dnd 3.75 in my mind. It also gives a nice expansion of options that really rounds out what both players and dms can do with the game and represent a number of character tropes that are difficult with just the core rules.
So if I had to choose one book to go with after the Core book, the Bestiary and the game mastery guide, it would be the APG.
2 core rule books? There's the Core Rulebook, what's the other one? Bestiary?
I second Kolokotroni's suggestion of APG and his reasoning. While I had been interested in Pathfinder since the Beta and had the Core Rulebook for "one day I'll try playing it", they weren't really that far off from D&D 3.5 for me to bother switching yet. By that time I was looking over D&D Essentials (basicallu awkward tentative transition step to 5E IMO) I was fed up with classes getting basically slapped on a new coat of paint and called new (the witch in essentials was practically the same as a wizard). The moment I read APG afterwards with its well built and flavourful classes of witch, inquisitor, and alchemist I had my entire group of friends switch over from 4E without a second thought. They resisted at first (edition changes always have resistance) they fell in with Pathfinder pretty quick after looking over the APG too.