Spellbook Adept

You have been purchased a spellbook, and your innate magical knowledge allows you to use it. For every level after you take this feat, that you take in the Arcane Trickster subclass, you may copy a single wizard spell into your book. The spell can be no higher than 1 level what you can currently cast, and must be studied and practiced for a full week before it can be used.

The costs are as follows:

1st level spell: 250gp

2nd level spell: 300gp

3rd level spell: 500gp

4th level spell: 750gp

5th level spell: 1200gp

You gain 1 additional spell slot that coincides with the spell learned, and regain this spell slot on a long rest.