If the book is divided into chapters, you have a natural place to start. I have the first chapters of EEL'S REVERENCE and FORCE OF HABIT posted on my web site.
When I visit someone else's blog and he or she or they want an excerpt, I'm most often asked for something much shorter.
Now, even if your book is NOT divided into chapters, it's divided into scenes, and most chapters are divided into sets of scenes. Your scenes are divided into "beats": pieces of scenes in which points are made, information is gained, character or relationships are revealed or shifted.
How many beats or how long a beat you provide depends on your length requirements. You might just want a one-line quote, or a variation of the quote like this "tagline" for FORCE OF HABIT:
All she wanted was a breath of fresh air. Was that too much to ask?
You might want a beat out of a scene:
"You are aware of the training vessel we operate in conjunction with the Galactic Union Space Troopers? St. Gregory The Wonderworker?"
"The kids call it 'Uncle Gus.'"
"Call what 'Uncle Gus?'"
"The ship. Galactic. Union. Space Troopers. GUS."
"Ah," Mother Hadrian said, as if the sound might pass for a display of interest. "Well. The students aboard are referred there by their teachers. They're the best of the lot."
Sure, they are. That's why they're out there, and you're down here.
"You have a challenging and rewarding..." Mother Hadrian referred to Bel's contract... "five years ahead of you." She thrust a hand forward, cocked for a blessing.
Bel intercepted it and gave it a bitter shake.
Slain with the jawbone of an ass.
Although every beat of your book should lead from what comes before to what comes after, be scanning your manuscript for more-or-less self-contained bits that can be lifted out and used to tempt potential readers.
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