ANYTHING AND EVERYTHING created before 1923 is in the public domain in the United States. That's just a fact. Fu Manchu is definitely public domain in the U.S. However, Sax Rohmer was an English author who died in 1959. In England and most of Europe, copyright lasts until 70 years after the author dies...especially if the author is European (I'm not sure if the law applies to American authors...I don't think it does). Therefore, while Fu Manchu is PD here, you can't print a book in Europe that contains the character, until January 1, 2030. This is the same reason that the League of Extraordinary Gentlemen movie couldn't use the "real" name of the Invisible Man...because it was released internationally and that wouldn't fly in England since HG Wells died in 1946, making Invisible Man under copyright there until 2017.
Keep in mind that a lot of smart people haven't done enough research to properly understand copyright...so just because they print something in a book, doesn't mean it's true. I still find that an amazing number of people still can't tell the difference between copyright and trademark. It apparently confuses a lot of people, and mistakes can be made both ways. Look at Dynamite...they apparently didn't do their due diligence on Green Lama. A simple wikipedia search would reveal that he was a pulp character before he was a comic character and that counts! So, don't always count on the "professionals."