Do stockpiled old mastery books have any use anymore?

[Phoenix Wright]

So I have a few mule inventories full of stockpiled mastery books and I have a few questions about them.

Can they still be used?
Is there any place I can do an exchange of some sort? There are a couple options with Maple admin but they're only for specific mastery books like Intrepid Slash etc.

[SaptaZapta]

They still work (unless the skill they're for was removed, obviously). You can only exchange the ones that don't, with Maple Admin.
They might be worth some meso still, probably there are people who would pay a mil or three for a 70% or 50% book rather than 7m or 10m for the 100% one.