There are two special types of resources for researchers who'd like to learn more about a particular treaty. These are treaty commentaries, and drafting-history documents (travaux préparatiores).
Treaty commentaries are somewhat analogous to annotated codes. They point you to books, articles, or other documents about the treaty, to drafting history, and to resources on interpreting specific language. Consider using a treaty commentary when you need in-depth information on the background and possible meaning of a treaty's language, or when you need some ammunition to argue for your preferred way of reading specific treaty language.
This PDF guide explains treaty commentaries and lists the commentaries available at the Zief Law Library.
Try these sources if you're looking for the drafting history of a treaty.