For anyone who needs help:
The book is unlocked with a key in the window of your room.
Once you've started stirring two different ingredients, you can only stop by finishing the potion (have the bellows pumped while the bar's in the yellow, moving or no), or overflowing it (and getting a side effect).
Two stirs and spamming the bellows is the fastest way to finish potions without much risk of overflowing, and can keep pace with the final day.