Seegee:
I don't think you can trademark names, especially when it's a common one like Harry Potter.
You can't get a blanket trademark on it, but you can trademark it in regards to action figures and other toys, and any other spinoff merchandise you can picture.
It's like how McDonald is a common name, but you even if your name is McDonald you can't open a restaurant called McDonald's because someone else has already trademark that name in regards to restaurants. For that matter, you can't serve McSwill or any food starting with "Mc" because McDonald's lawyers will (and often do) argue that you doing that may confuse a consumer into thinking that you are somehow connected to the McDonald's chain.
Personally, when I reread that book the name Harry Potter jumped out at me - even though I had barely noticed it the first time I read it. Because Harry Potter is a household name, any book that uses that name will jar the reader.
Maybe 10 years from now it will be different.
Goodgulf