So... just making sure i understand this...
P.57 Ranks. Indicates how powerful a spell is. But has really no other effect mechanically, other than you start with Rank 1 spells.
You can hold a number of prepared spells = to the base chance of INT (p. 25). Your spells are stored in your grimoire. So... the moment you "learn" a new spell that goes beyond your base chance limit... you HAVE to have you grimiore to record the new spell in (otherwise I'm assuming you "forget" one spell to hold the new one... like if your lost your grimiore. If you lose your grimoire, im assuming you don't lose the spells you had prepared... like after you run out of will power... you just can't swap spells out.
Power Level... this is how much WP you put into the spell based on its power level range of 1 (magic tricks) 2 (most other spells) or others that cost 0 or default 2. And power level has nothing to do with Rank.
If a spell has a power range of 2... it cost 2 WP to cast or, if the spell allows, for another 2 WP you can bump the spell up with effects described in the spell (increased range, damage, duration, etc).
That about right?