Ok, I'm probably missing something very obvious here but I'm tired, so please help me out!
Here is the feat as written:
"Spell Mastery: A wizard (and only a wizard) can take the special feat Spell Mastery. Each time the wizard takes this feat, choose a number of spells equal to the wizard's Intelligence modifier (they must be spells that the wizard already knows). From that point on, the wizard can prepare these spells without referring to a spellbook."
I have two questions that seem to fall into gray areas in the rules, IMO.
#1. If I previously lost my spellbook (assume I haven't had the option to replace it yet), can I pick a spell from what spells I know from that book as one of the spells for Spell Mastery?
While I no longer have access to my spellbook, nothing about the description in Spell Mastery says I still need to have access to it in order to make it one of my Mastered spells. The last sentence might hint towards that, but it’s circumstantial.
#2. If I study a spell from another wizard’s spellbook and successfully learn it, do I have to first copy that spell into my own spellbook before taking it as one of the spells in Spell Mastery?
With this one, there would seem to be a small amount of time between sitting down and learning someone else’s spell from their spellbook and applying my new Feat. Would this be logical? I’m sure there is something in the PHB that says you have to apply the Feat immediately, but what if I learned the spell from the other spellbook and just never copied it down, keeping it in memory until I leveled up?

Here is the feat as written:
"Spell Mastery: A wizard (and only a wizard) can take the special feat Spell Mastery. Each time the wizard takes this feat, choose a number of spells equal to the wizard's Intelligence modifier (they must be spells that the wizard already knows). From that point on, the wizard can prepare these spells without referring to a spellbook."
I have two questions that seem to fall into gray areas in the rules, IMO.
#1. If I previously lost my spellbook (assume I haven't had the option to replace it yet), can I pick a spell from what spells I know from that book as one of the spells for Spell Mastery?
While I no longer have access to my spellbook, nothing about the description in Spell Mastery says I still need to have access to it in order to make it one of my Mastered spells. The last sentence might hint towards that, but it’s circumstantial.
#2. If I study a spell from another wizard’s spellbook and successfully learn it, do I have to first copy that spell into my own spellbook before taking it as one of the spells in Spell Mastery?
With this one, there would seem to be a small amount of time between sitting down and learning someone else’s spell from their spellbook and applying my new Feat. Would this be logical? I’m sure there is something in the PHB that says you have to apply the Feat immediately, but what if I learned the spell from the other spellbook and just never copied it down, keeping it in memory until I leveled up?