I own a bunch of PF 1E books. Most are very good RPG books. I don't know if I can differentiate between rule books, and source books? I am not much on RAW, anyway.
The NPC Codex, and Monster Codex are favorites of mine.
The Bestiaries are top notch. I own numbers 1, 2, 4, and 6.
The Advanced Player's Guide compliments the Core Rulebook well; and the two volumes successfully provide a PHB and DMG.