Arcane Synthesis

Pooling your knowledge of Spellcraft, you master an additional Wizard spell of your highest level spell slot. This Mastered Spell is always prepared and requires spell slots to be cast.

During a short or long rest you can copy known spells from other sources into your Wizard spellbook at no cost, as long as they are wizard spells that are always prepared and are of a level you are able to prepare. If your Mastered Spell is copied into your spellbook, it is no longer always prepared and a new Mastered Spell can be learned. You can only learn a spell this way once per level.