Codex Seraphinianus and the Voynich Manuscript come to mind. Both are only really practical as digital downloads, as the former is highly collectible and the latter is a centuries old one-off handwritten tome. Both have the advantage of not being based on any known real-world religions (unlike, say, the Key of Solomon).

I've never played the game, and the sheets give me the sense that it probably wouldn't be my cup of tea... but Eoris Essence's sheets are beautiful in a way that most character sheets really aren't.