It doesn't make sense to think of it as a direct object. A direct object is a thing that receives the action of the verb. Like if you say, "I read the book", "the book" is the direct object because it is the thing being read. "Sally answered many questions", "many questions" is the direct object because it is what is being answered. Etc.
There may be cases where the idea of "receives the action" may be fuzzy. But I can't think of any sense in which "about the book" could be said to be receiving the action of "told". You could say, "I told my friend the title of the book." Then the thing that you told is "the title". But you wouldn't say that the thing that you told is "about the book". Maybe what you told is something about the book, but for that to make sense, there has to be a "something" in there.