Personally, I'd like a book on constructs. As I like to have the presence of encounters in dungeons make sense, I use a lot of undead and constructs as guardians in tombs (I have trouble with groups of intelligent monsters living for years in someone's tomb without ever having found the secret doors or treasures of the place).
On undead there is plenty of stuff (what with my old ravenloft collection, the Libris Mortis etc.), but on constructs there is very little AFAIK....