XPH, and The Complete Book That Shall Not Be Named have some content (Illithid Slayer, at least for XPH, and Illithid Heritage Feats for the second), as does Dragon Compendium (Illithid Bloodline), FR-Underdark (Illithidwrought wp and armor, Half Illithid, Illithid grafts, etc)
[edit]Savage Species has a little[/edit]