Dear Miss Snark,
The agency I would like to query says clearly on their website "...nothing should be enclosed with the letter; don't send any sample material until we advise you that we'd like to see it." So, for this case, I should not attach the first few pages?
I see an almost zero correlation between enticing query / cover letters and enticing writing so I always want to see actual pages.
However, if the guidelines specifically say "send nothing" I have to advise that you send nothing.
They must be better at scenting good writing from cover letters than I. I'll blame the gin.