I like Setting Info to be predominant in any book - it has to have flavour all it's own to intrest me. I like Tolkien-esque stuff, so if i notice some of that coming off, i will like it more, unless it is terribly written.
Rules come next. I need to know if there are any changes to standard d20, and determine how that affects the charcerter i or my players will be making. New races, classes, feats, spells and such would be high on my 'to see' list. Of course, these will tie in with the campaign feel, so it's a give and take thing with that stuff.
Fiction, i could care less for. If i want fiction, i will buy a book, as someone aptly put. Give me a bit, perhaps a papagraph or two in th beginning of each chapter and interspered as sidebars and such, perhaps have a few creation myths of something in chapters dealing with the nature of the world, but for the gods' sake, don't drown me in text that i will have no use for.
Iron Heroes, as an example, was very enjoyable for me. It gave me the impression of a low magic campaign setting, but gave me MANY rules to create a charcter in that sort of setting. I would have to say it was more Rules (1), Campaign (2), Fiction (3), but i liked it. It helps that ther is some amazing artwork in that book; i would have to say that is the one thing this survey is missing.