Here’s something I’ve been known to say more than once at writers conferences:
I need 50 pages to know if a book is great, but only one to know if it’s not.
(Disclaimer here about the fact that “great” means something different to everyone.)
I can decide the fate of a query in 60 seconds. So can most other agents I know. Sometimes the query describes a kind of book I generally don’t enjoy reading, or I can’t get a sense of the plot, or it’s got some other quality that tells me the writer and I won’t be a dream team.
If a query is intriguing enough to get me to read the sample pages, that’s where I’ll really know whether the book is something I want. As I’ve mentioned before, I can never be quite sure what I want in terms of plot, concept, genre, etc., but good writing can appear in any genre. What’s not good writing in any genre: clunky dialogue, inauthentic voices, characters I don’t care about, etc. And anonymous dead prostitutes in mystery or thriller novels. When I see those things on the opening pages, I pass on the query because I have yet to see one that overcomes those elements in subsequent pages.
A good book can’t be built in one page, though a great opening page can set the stage for one. Good books need time to develop. Plots, characters, and problems have to be introduced. None of this can be done in a single page, but by reading 30-50 pages I can get a definite idea of the direction the book is taking and decide if I want to pursue it further. It’s possible that I’ll give up on a book after those 50 pages, but it’s still a sign that the opening pages showed promise.