++ to Scribus. LaTeX and friends are dynamite for writing documents (I use LyX when I can to help deal with LaTeX, but do all my articles and presentations with LyX/LaTeX), but they aren't what you want to use for pretty graphic layout. When I have to do posters for conferences, it's Scribus all the way (with LaTeX for equations), and I'd imagine if I was doing a game manual, you'd want Scribus. You CAN do anything with LaTeX, but it's generally not worth it for graphics-heavy things like nice manuals.
It can be a bit opaque, but there are some great tutorials out there.