Ars Arcanum (Latin for "The Art of Mystery") is a glossary or appendix section at the end of the books that are part of the Cosmere universe. The information is presented from an in-universe perspective.
Ars Arcanum in each book or series
All of the cosmere stories containing an Ars Arcanum are listed here in rough chronological order.
The books in the Mistborn series each have an Ars Arcanum with the table of Metallic Arts, describing what each of the Allomantic metals does. Since more metals are discovered and new things about them are learned, the table in each later book has more metals and information with greater accuracy.
The Stormlight Archive
The Ars Arcanum in The Stormlight Archive gives detailed information about the Ten Essences, as well as descriptions of the different Surges and how they are used by Surgebinders. Some information has been added in each book: a Lightweaving section appears in Words of Radiance and Oathbringer contains an additional section about Soulcasting.