Benefit: Each time you take this feat, choose a number of spells that you already know equal to your Intelligence modifier. From that point on, you can prepare these spells without referring to a spellbook. Source (SRD)
The only benefit Spell Mastery gives you is the ability to prepare a selected number of spells without access to your spellbook. You still need to spend the usual time to prepare spells.
"Each time you take this feat" implies you may take the feat more than once, and again select a number of spells you know equal to you intelligence modifier.
It is worth noting that in Pathfinder benefits from your ability modifiers are retroactive, so if you take the feat at an Int bonus of +2, level up and increase it to +3, You may choose 1 more spell as per the feat. If you were to then take the feat again you would choose another three and hence be able to prepare a total of 6 (specific) spells without your spellbook on hand.