As far as I know there are no book bundles. How the books relate? I'm not sure I understand what you mean but here is a short summary of the content of the main books.

Core = Players guide + half DM guide (the basics of dungeon creation and magic items section that usually is in the DM guide in D&D), basically contain everything you need to play the game.
Bestiary I = Monster manual. You only need this if you are a DM and don't want to look stuff up on the net.
Game Masters Guide = More tips on world building. You can probably skip this if you have a similar book from D&D as it deals mostly with fluff. It also contains some rules but, you can usually just wing it, don't bother looking them up when they are relevant once in a blue moon.
Advanced Players Guide = More classes, feats and spells. Probably your best pick for a 2nd book as a player.
Ultimate Magic = Mage stuff. Alternate class features, spells and feats mostly. Contains the magus class (gish base class).
Ultimate Combat = Martial stuff. Alternate class features, feats, mundane gear and some spells. Also contains the gunslinger class if that is your thing.
Ultimate Equipment = LOOT!!!
Advanced Class Guide = More classes and alternate class features. The classes in this book are thematic and mechanical blends of classes from the other books. Some cool feats and spells.
Advanced Race Guide = More options for the core races plus a lot of races that were previously just footnotes in the bestiary.
Bestiary II-IV = More monsters. A lot less valuable than the first bestiary because the monsters are more niche. I recommend the Monster Codex instead.
NPC Codex = Premade NPCs with levels int the core classes and prestige classes.
Monster Codex = In depth info on classic monsters including premade monsters with class levels. Probably the 2nd best monster book after Bestiary I.
Mythic = Silliness. You are better off without it.