Whoo-boy. This is a can of worms.
The best recommendation is to look closely at what each agent or editor says and follow their guidelines. Some say "query only." Others ask for a specific number of pages (usually five or ten), and those who operate on an older (more sensible) model usually want what was traditionally considered to be a "partial:" the first three chapters, or the first fifty pages.
Problem #1: Some agents don't specify a damn thing.
Problem #2: Many specify "query only."
How you approach these problems--and I say this with full awareness that I will probably get flamed--depends on how effective your prose is. Some agents (Miss Snark among them) say that "writing trumps all" and think that at least few opening pages really ought to be part of a query. "Query only" is different in the minds of some agents from "Query letter only."
In a sensible world, it is hard to imagine that any agent would be offended by a few pages. The worst that could happen is that they would toss them in the trash--Lord, what a burden! But, suprisingly, some agents use "Query Letter Only" as a sort of litmus test, and claim that they automatically reject any writer who sends anything material but the query letter itself (or, if specified, Query + Synopsis).
The few who have made statements as to why they would automatically reject anything that included unasked-for pages say that it is a test of author compliance, to see if we can follow the rules.
I always included pages, because I knew that if anything was going to sell the agent I wanted, it was the writing. Did I get reqests for manuscripts from agents who had stated "query only"? I did.
Did I get rejections because I included unasked-for pages? Possibly (how would I know?), but the fact was that any agent that worried about my "client compliance," who hunted down manuscripts solely on the basis of the TV-Guide summary included in a query letter, was probably not the agent for me. This "testing" idea works both ways.
{Space inserted here for forum members to shout, "Always, always send exactly what the agents asks for, no more, no less!"}
As to what happens next if the agent/publisher is interested, they will generally want to see either a longer partial (three chaps if you sent only ten pages, for example), or, more likely, the full manuscript.
Were I you, I would have finished the manuscript, polished it, gotten feedback from readers, and rewritten before sending out even partials. You do yourself no good and much harm by shopping a manuscript that isn't ready.
What's the big rush?
