Is there any books out there that details old spells and potions?
Of course, since spells and potions don't exist in this world, all we have are sources from people who believed they existed.
For spells, you could check out the Malleus Maleficarum. It's a medieval treatise about witchcraft, mostly written for inquisitors and theologists. In the second section you may find some hypotetical description fon how witches did their spell and rituals, or at least it should give you an idea on what the clergymen of the times believed witches capable of.
For potions, I'd give a look on historical figures - alchemists. Often seen as something like early chemists, alchemists were borderline scientists and magician, and have left a lot of written traces about their procedures and beliefs (e.g. the famous myth of transmuting gold). Among them, you could check out George Ripley, the alleged author of the somewhat famous Ripley's scroll, which describes, enigmatically, the procedure to create a Philosopher Stone.