Book ownership, enchantment tiers and the published cost schedule.
Books remain owned after enchanting and can serve several items. Removing an enchantment refunds its Enchantment Points.
Vendor upgrades cost 30 then 60 Echo Shards, opening tiers II and III. The level is shared among the five locations.
The listed tier-I point costs are 1/1/2/3 for Common/Rare/Special/Unique, with additional-tier increments of 1/2/2/2. The calculator labels its interpretation; the reference does not fully specify repeated service-charge behavior.
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Published source · Wikily · Numerical facts and original explanatory summaries. Not independently tested in game. Unknown formulas remain unknown.