I was a little confused by the title, but later the article clarifies that the intent of little free libraries is to 'plant' (i.e. donate) books: Little free libraries are put at the curbside by homeowners and are an invitation for anyone to leave and take a book.

So I'm not sure what the police is investigating - is the (unnamed) book itself banned in Canada?

Whoever find the book should substitute their own book instead. This free library is meant that someone can put a book or take one, anyways. It should not be the thing for the police to get involved (unless further worse things happen, other than putting in or taking books), I think.

Whats the name of the books? Didnt see them in the article.

Supposedly: https://books.google.com/books/about/?id=s1pLvAEACAAJ

I would have just removed them and moved on, but starting a police investigation and publishing a news article about it is certainly another approach.

Writing news articles about this behavior just glorifies it.