With the new power edge it is for a Power of your Rank or lower, yes.
So in our rules a slot of that rank.
BUT I would also alow you to add a new spell to your spellbook also of that rank when you take the New Power Edge.
Sounds generous right? B/c it is ;) But don't mind. I really didn't have to worry with others as I was converting. But am happy for you to do BOTH for the purchase of an Edge.
So you start with 3 Novice 'Spell Slots'.
If you take New Power Edge at Seasoned rank you also now have a Seasoned Spell slot. So following each extended rest you can prepare 1 spell from any of Seasoned in book. (1 of which came as a bonus with this Edge choice - others could have been amongst those = to Smarts).
Basically - Spellbook should have a # of spells = Smarts die (of ANY rank) + 1 per time the Extra Power Edge was taken. (Happy for you to use current Smarts die too, reflecting the few extra spells you may have added over the years).
Not sure how Phtunk matches this, I simply converted the main things he already HAD, but it sounds reasonable going forward (for a starting book)?