The Codex Immortalis is the definite collection of Mystara's Immortals. My friend Marco wrote these years ago in Italian, but thanks to Mystara enthusiast and Piazza member, Mortis, this two volume pdf is now available for free download for the first time in English.
Not only are these books packed with information, but they are also beautifully presented. Now I know what I will be reading this weekend! Go here to see how you can download them for free.