I feel like an idiot asking this question, because it should be obvious to me:
I'm about to add some XP into Spell Mastery for the first time using the 5th ed. rules, and I want to make sure I understand the RAW. If I want to throw one point into DEO, for example, I gain a "DEO Mastery" ability of 1, correct? I don't need a book to learn that ability, do I? Do I need to spend lab time to gain the ability, or can I simply use adventure experience? And I would use the ability advancement table from there to gain levels in the ability, correct?
Spellmastery is an ability not an art. You need to shove 5 in to get a score of 1. You dont need the lab and can add practice XP, expsoure I would guess in in cases, read a book on the appropriate mastery of bung in any story XP if you used it in the story.
Choose your mastery benefit carefully as mastery, being an ability, is hard to increase.
TBH unless the magus chose the mastery related virtues, it is not a common use of XP in our sagas. Saving spells very widely used in combat.
It's unusual that 5 xp be spent in any ability all at once in one season unless that had a specific study source (text or teacher, or possibly the ever-painful "practice"), but some Troupes wink at such limits.
For me, the most common Masteries are 15 xp into Multi + Penetration for some low magnitude/high casting total combat effect (3 w/ penetration, kkthnx!), multi for a ReCo "Leap" spell to get the hell outa dodge w/ at least one other, and possibly a FastCast with a multi-purpose defensive spell (again, like "Leap", or perhaps a big stone wall or something).
While some combinations of multiple Mastery sound great on paper, as RK points out they rapidly become expensive, and putting those points into Arts or Magic Theory, or another level of Artes Liberales and Dead Language: Aramaic, or Profession: Omphaloskepsist becomes easily more attractive. For my money, at least.