Ok there is that book with all the girls on the cover combinied... does that book matter in the series?

It's the Night World Ultimate fan Book. It tells tu all about the Night World. It even has cuestionarios that tell tu what character tu are. There's even the first 2 chapters of Strange Fate in there.

I just got The Ultimate fan Guide from amazonas, amazon it's awesome so if you're wondering if tu should buy it - yes buy it .

the fan guide is cramed with facts youll b a night world know it all

It' doesn't have any continuation for the series other than a little excerpt for Strange Fate. It's just the guide, with quizzes, facts, and stuff.