The Halloween Documents, so-called because the first one leaked in October 1998, don't actually have much to do with Halloween itself—which I find sad, as an avid fan of the holiday. But for understanding the historical relationship between Microsoft and open source, the memos are vital.
They were the first to reveal the particularly nasty "tricks" Microsoft planned in its effort to contain the open source movement, and to prevent Linux in particular from cutting too deeply into its revenue. One key strategy for the company was implementing proprietary protocols to lock customers into Microsoft software. Another was touting Microsoft software as offering lower total cost of ownership (TCO) than Linux, even though the documents showed that Microsoft itself found Linux to be the cheaper overall solution in many cases.