The extra 900 or so extra mag def can't hurt can it?
As for which books to get, its up to you. You can either get all 3. Or 1st and 3rd, or just the second. The second just gives you a percentage of block on all mag attacks.
I personally have the 1st and 3rd book, only have the 1st skill of the 2nd book. But I'm a pure STR glavier, so you may have different needs to me

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Ooh, I got a sexy ex-staff title!