Does the Evocation Savant feature apply to evocation spells copied into backup spellbooks?

Question

The normal cost of copying a foreign spell into your spellbook is 2 hours and 50 gp per spell level. If you are copying spells from one of your spellbooks to another of your spellbooks (to keep backups) then the cost is reduced to 1 hour and 10 gp per spell level.

The Evocation Wizard feature Evocation Savant halves the cost of copying evocation spells into your spellbook. I expect this to apply to copying foreign spells into your spell book (having the cost to 1 hour and 25 gp per spell level), but does the feature also apply to making backup copies of your spellbooks (cost of 30 minutes and 5 gp per spell level)?

Answer 1

Yes, the feature applies to all spellbooks you own. The game rules don't distinguish between main and backup spellbooks, so they are all your spellbooks.