So... Are bought they by Art or Abilitiy cost Book?
I believe they are bought as Abilities.
And Arts like the Goetics or Phabulosa?
Are clearly Arts 
As dar as I understand, that's really the only indicator.
Ok, many thanks!
I would advice against actually using the ability book rules with accelerated abilities.
Sure, it's technically the Rules As Written, but it produces quite silly results.
Ars Magica is not a game where you're required (or even intended) to play everything precisely by the Rules As Written.
If I remember correctly, the Goetic Arts are actually Accelerated Abiltiies. They're just called "Goetic Arts", but mechanically they are described as accelerated abilities. Just to confuse things even more, I guess.
Regardless,
+1
I'd recommend using the Art costs for Accelerated Abilities and the Ability costs for Difficult Arts. Or just set a higher build-point value to suit how you feel the book should cost.
All this questions are for Ex Miscellanea and Corrupting Goetic Books on the stories. Not for general use on my game.
And of course many thanks.
Hermetic Ars are accelerated abilities themselves, so I would use Hermetic book costs for other Accelerated abilities as well. No problem.
Not simple...
Arts use a 1 3 6 scale, with a +3 from puissant.
Abilities use a 5 15 30 scale with a +2 from puissant.
You have accelerated abilities, which use 1 3 6 scale with a +2 from puissant (not sor sure about Ars goetia, I would have taught about the Runes of Viktir, but it has been 2 full years since I read that book)
You have (forgot the exact name) slowered arts, such as the Elemental forms IIRC, which use a 5 15 30 scale, but with a +3 from puissant.
Arts use generally a Charact + Art +Art + die, where abilities use Charact + Abilities.
Then you have Method and Powers. Which are abillities IIRC, with +2 from puissant, but a mechanical use like arts.
Have fun!
That just shows inconsistency in the puissant bonuses, a failure of the rules not of the concept*. Adding puissant to the mix messes things up. I think that using the XP scale is a fairer approach here.
Xavi
* having 2 advancement methods and scales of power is a rules failure, but that was introduced with Arts back in 1st edition, so is nothing new. IMO the game would benefit of having everything use the Arts XP scale, but that is me.
...and I'm still against "Arts based on XPs", but that's just me I guess 
Slowing the progression of Abilities makes sense in a game about magi.
It is just a thing about the scale, not true power. And does not make much mathematical sense IMO. Having an ability at level 4 or level 9-10 can be exactly the same as long as you change the reference base.
Xavi
Yes and no. I'll come back to this later in the day when I'm not cooking.
As they use the same die-type it means that any roll using Arts is more dependant on learning, while any roll using Abilities is more dependant on luck.
Rolls with Arts, with the exception of Certamen use two Arts, so they magnify the difference between Arts and Abilities.
As others have said, RAW is perfectly clear. Accelerated Abilities are abilities in every way except for the cost to level. The build point cost is for abilities. They can be trained like abilities. When you right books they are written as ability books. All that said, you may want to House Rule it.
Ok. Thanks.
I think that the Ability-like Cost for that Books make them valuable with the thinking to give them on Ex Myscellanea or Apprenticcces to make them learn it and maintain them on some extend. Or to use like investigation Texts to Integration proccess.
Which was how things worked in 4th ed. Both Arts and Abilities used the 1 3 6 pyramid scale, however Study Totals for Abilities were /5 where Arts used the full amount. This led to some odd results regarding rounding up or down (I can't recall the exact rules). So sometimes the Quality of the book read did not matter much since you'd be rounding down all the same. And this was avoided by 5th ed's usage of two different scales. I, for one, like this better.