Originally I had thought to create a kind of wiki, where I could stuff all the, um, STUFF. The research notes and visual references, sure, but also all the other bits I'd accumulated over the initial writing: quotations, musings, fragments of period and current texts, random images, songs, late night emails to myself ... at various points I've used Evernote, Tiddlywiki, Tumblr, Pinterest, good ol' paper notebooks, and even a 3-ring binder. I've been thinking about this series off and on for 10 years. Things have piled up.
As I started organizing the material, however, I realized that much of it is book-specific and can be tied to each volume—annotations, essentially. Those posts are under the book title. The other material I've split into the following categories:
Behind the Curtain – this is stuff related to my writing process and what it looks like from the backend. "Pay no attention to the man behind the curtain!"
Below the Waterline – this is big picture worldbuilding stuff and thematic influences. The name comes from the iceberg theory of worldbuilding, where only 10% of your work, the tip of the iceberg, makes it into the story directly; the other 90%, the rest of the iceberg, is below the waterline—out of sight, but allowing you to write with the confidence of a fully-realized world.
Characters – a few different lists of all the characters in the series and the various books they appear in. Believe me, I need this as much as you do.
Language – book-specific glossaries of the Sereidenikè language, as well as some choice tidbits of 18th century slang.