I present you an entirely worthless and random question... Perfect for GD.
Who do you think the best mathematical authors are?
I've got two nominations:
1 - Serge Lang - Sure, some of his books are nearly incomprehensible, but there packed full of information. Sure, he may redefine the term "obvious", but I'm still rather fond of his style. I particularly like his book, Algebra.
2 - Klaus Jänich - A truly remarkable author. He presents complex topics with great clarity, and in a manner that truly helps the reader learn. A quote from the preface of his book, Vector Analysis (the English edition): "An elegant author says in two lines what takes another a full page. But if a reader has to mull over those two lines for an hour, while he could have read and understood the page in five minutes, then - for this particular reader - it was probably not the right kind of elegance." Almost like he was talking directly to Lang in that passage.