"Eu quero tratar do livro."
I'm a brazilian and I never use "tratar" to talk about book. It's too formal, and it's not common to read or to listen something like that. A better translation could be "quero falar / discurtir / conversar sobre o livro". Depending on the situation, "tratar" could sound like "take care of the book".
Interesting. So although a book can treat/discuss a subject, I guess that's a legitimate use of "tratar", you would never use the word to mean discuss the book. How would you feel about "I want to deal with the book" as a translation? (See: http://dictionary.reverso.net/portuguese-english/tratar) and If you are happy with that, then it's not a big leap in English from there to "I want to discuss the book".
"Tratar" used as "discuss" is right, but not common or popular. Well, I'm not a native english speaker, so "deal with" for me is used as "deal with a situation". So if I need to "deal with a book", I understand that I need to read it, or something has to be done with it, but not exactly discussed. But I can be wrong about it.
Thanks for your answer. I always seem to be quoting dictionaries in our mutual discussions, so forgive me for doing it again. See item 3 point 4 here:
I think I'm just going around in circles trying to make Duolingo's translation fit, when it clearly doesn't. So given this exercise is to translate "Eu quero tratar do livro", and not to rewrite it in better Portuguese, what would your final answer be?
In this situation, "the novel deals with several different topics", "deal" can be translated as "tratar": "a novela trata de diferentes tópicos". It means that the novel brings different topics, but "tratar" can be replaced by "aborda", "traz", "cita", "discute", "fala sobre" and many others, but as you can see, not only "discuss". But "discuss" is also acceptable. I just want to expose that "tratar do livro" is too formal in Brazil, and "talk about" is more usual.
Someone correct me if I'm wrong, but I'd say "tratar" is used here as "discuss" in a more formal sense. Like, you're talking about a book in class, or writing an essay, rather than just talking about it with your friends. You'll see "tratar" used to introduce subjects, as in "este livro trata da economia" ("this book is about the economy").