One of the standard pieces of advice that writers are given is "Write the book you want to read." This is great advice, but like all advice it has just as much potential to leave the recipient hidebound as it does to liberate him. It's no fun gnawing through straps you pulled into place yourself, trust me.
First, the good parts. It's always best to write the kind of book you want to read, because that increases the chances of you writing something that doesn't actually exist yet. If the kind of book you want to read can't be found on shelves, and you go through all the sweat'n'toil of bringing it into existence, that's a net win. You, and everyone else, get to enjoy the spoils. But ...
(Digression re: minor irony at work. Most of the authors I know, self included, have little motivation to go back and re-read their own work. If Vladimir Naobokov once compared showing people unfinished writing to passing around samples of one's sputum, one could analogize pawing back through one's own work as licking said sputum up and attempting to assess its vintage. End digression.)
... as I was saying, but drop back ten and punt for a sec. Look at those words, "the book you want to read". I don't know about you, fellas, but every time I'm in a bookstore and I'm not looking for a specific book, I'm swirling in a Sargasso sea of heady indecision. Most of the time I have no damn idea what I actually want to read, in the sense of it being an affirmative. To wit: I am fairly positive I do not want to read a book about a giraffe lowered the wrong way into an industrial sanding belt (although judging by some of the High Weirdness By Email that arrives in my in-box these days, there are people out there busily making Animal Grinding Fiction into a subgenre).
Most of the time, no, I don't have any specifics. I just know I want something that stands out by dint of not being a copy of a copy of a copy, of not being something I can affix a label easily to or one which already sports such an easily affixed label. nd therein lies the trap I hinted at earlier, though: if you don't know what you actually want — in an affirmative, specific, positive, discriminatory-in-a-good-way sense — you'll never be able to ask for it. In fact, forget about asking for it from anyone else, you'll never be able to ask for it from yourself.
"Write the book you want to read" also requires that you know what kind of book you really want to read. Not easy. On top of that, it requires that you know what you want, period, and you know how few people even seem to have any idea of that lately? It's a tough enough dilemma to grapple with when you're just a fan / consumer, but it's a buttkicker-and-a-half when you're a creator.