The Affair of the Poisons was a murder scandal in France during the reign of King Louis XIV. It launched a period of hysterical pursuit of murder suspects, during which a number of prominent people and members of the aristocracy were implicated and sentenced for poisoning and witchcraft.
The sensational trial drew attention to a number of other mysterious deaths, starting a number of rumors. Prominent people, including Louis XIV, became alarmed that they also might be poisoned. He told Gabriel Nicolas de la Reynie, who, among other things, was his chief of police, to root out the poisoners. The subsequent investigation of potential poisoners led to accusations of witchcraft, murder and more.
The next stage of preparing an outline is do some research. I need now to spend some time immersing myself in the period and the details of the Affair of the Poisons. Research, for me, begins with Wikipedia. This is perhaps a controversial position, but as long as iresearch doesn't also end there, I don't see a difficulty with it. Wikipedia may not be original scholarship but where it is excellent is in making links between related fields of knowledge. A free-wheeling hour on Wikipedia can be very helpful both to gain a thematic overview and to extract some telling detail. In this case, Wikipedia was very helpful in drawing my attention to the Fronde, a kind of covert nobles' coup in Louis XIV's minority--outside of the timeline of the story but clearly important in terms of how the court, and Louis, viewed themselves a generation later.
We move next to some more detailed reading, both specific and general. The starting point here is Anne Somerset's The Affair of the Poisons, the standard English-language popular history of the events. I've read this before and re-reading is now in order. I've also summoned up two biographies, Louis XIV by Vincent Cronin and The Loves of Louis XIV (Antonia Fraser). This gives me two different perspectives on king and court. Finally, I've also acquired the only English language novel I'm aware of on the subject, The Oracle Glass by Judith Merkle Riley.
Many would advise against reading other novels treating your intended subject. I take the point, but from what I know of the book, its preoccupations are rather different to mine; I hope it will illuminate rather than constrain my imagination.
Some of my more informal research included speaking to real live French people. There was a clear consensus here that if I wanted to cover this period, I needed to read Dumas' The Man in the Iron Mask, and for various reasons this seems to me a worthwhile pursuit.
That's my summer reading taken care of. Note that at this stage I am not even close to knowing the plot (the Affair of the Poisons is too large to fit everything in). I don't even know the characters, or the mixture of historical and fictional ones. These are the things which will determine the flavour of the book, but at this stage in my knowledge, those decisions are premature.