I'm a bit new to Pathfinder and very new to spellcasting in particular (L4 wizard! woo!)
My question is thus:
If I take the Spell Mastery feat, can I cast those spells without preparing them provided I have the components? Or basically, do mastered spells count towards my limit of how many spells I can have prepared? If prepared spells are just ones I have temporarily memorized, it wouldn't really make sense for mastered spells to count against that, but I can't find a ruling on it anywhere.
From the sounds of it, you still need to prepare them as you would normally. The benefit is that you wouldn't need your spellbook to do so.
If you're in a situation where you lose access to your gear, or if something happens specifically to your spellbook, you can still prepare the spells that you selected through Spell Mastery. This way, you aren't completely left without spellcasting abilities in such a situation.
Spell Mastery lets you chose a number of spells equal to your Intelligence modifier that are already in your spellbook; you can prepare these spells from memory and don't require your spellbook to prepare them, unlike normal. It does not grant you the ability to cast spells without preparation or cast more spells than normal.
Basically, assuming you have a 16 Intelligence, you can pick three spells for Spell Mastery. If you were ever to lose your spellbook, you could still prepare these three spells without it, although you obviously couldn't prepare anything else because you don't have your spellbook.