I guess with me it is a number of things.
Usefulness - When I'm reading it do things in the book strike me as being able to be usefully applicable to real life, if they were changed around.
Associations and Allusions - I like allusions and associations to things like real life mythology, hosiery, religion, science (if applicable), folktales and legends, mystery, etc. I personally don't game to escape or divert myself from life, I game as a sort of extension of other things I do in life. So good strong associations are important to me. I have no interest in a game or a milieu or setting that is totally fantasy based, or divorced from reality. Rather I like an extension of reality in which it is possible to explore real world problems in a different way, setting, or context. I like a game that helps establish such criteria for problem solving, and I like a gaming book that is more concerned with overall usefulness than mechanics. (Though good mechanics are important, they are the easiest thing to me change and manipulate according to taste - it takes little effort to envelop better metahetmtcoial models than most games use, but a bad gaming philosophy means the whole game has to be redesigned to be useful - that is to say it is far easier to tweak numbers and formulae than it is to modify an overall bad design. Same with a book. It is very easy to change the numbers in the book, very hard to change a badly written and conceived book into something useful. Better in that case to just write your own and far better book.)
Interesting and Stimulating to Read - I prefer books that are interesting to read and multi-layered, rather than those that read like dry, anemic, and linearly designed textbooks with just a one dimensional application. In college I almost universally skipped reading textbooks altogether, for that very reason. If in psychology class I read Jung, not a textbook about psychology. If in physics class or mathematics I read the Principia, and skipped all that useless textbook crap, which was often filled with erroneous interpretations anyway. (It has often been my suspicion that many people who write textbooks never read the original work they are discussing, certainly not all of it, they just simply read what someone else had said about the original work. In other words many textbook writers make a living simply by writing books about other books. I often seem to see the seem general process at work in many game designs and books about gaming. Nothing really original written, no original methods of presentation either, just stock formula repackaged in a different form. More like a redesign of an old design in a different facade than a new paradigm being presented.)
If I was in religion or philosophy class I read scriptures or the Philokalia, or the Republic, or the Novum Organum. I hate textbooks (because for the most part they are as useless as they are often wrong) and I hate gaming books that are set up like textbooks. For that kinda stuff I prefer just plain old chart and graph books, and computer programs.
That was one thing I really liked about Gygax. Yes, there were charts and graphs and mathematical formulae (though crude) but they integrated pragmatically and practically into the overall work, they were not the overall work. It was more like reading an original Work or an Opus when Gygax wrote about gaming, than reading a stupid textbook or technical manual. Almost more like reading a piece of literature about the concepts and philosophy of role play gaming, or viewing a piece of artwork in motion, than reading a table of functions.
Interesting Methods of Combat, Wargaming, Negotiation, Maneuver, and Espionage. If it's gonna involve tactical wargaming then it ought to be interesting and challenging when it comes to combat, and to conflict resolution.
Innovation and Originality, or an original reinterpretation on an old problem
Good artwork and graphics. If I could I would have all really good gaming books be something more alike illuminated manuscripts than cartoon blocks.
With me, and overall, I think it is about integration.
If the game integrates with my overall life, and is not a sort of second life or just a diversionary enterprise, then I like that. I can usually tell by reading the gaming manuals (and I wish more game books were more like Gygax's original Handbooks and Guides than mere technical manuals about gaming operations and functions) if the game itself will be useful to me.