Legal Questions About Sports Writing

[MV64]

I've got a quick question for those of you who might know something about this:

I'd like to write a book about a professional sports league, but I'm not sure about all the various laws that would govern writing a book.

I know that sports teams, leagues, etc. are trademarked or registered, so would I need to gain written approval from them in order to write a book about them? Obviously I wouldn't pass the book off as officially endorsed or anything, but since anyone can write articles online about sports teams, I imagine anyone could write books about them, too.

Am I right in this?