Know when to stop: when you've given the reader enough to want to read on or decide that the book isn't for them.
It's a good idea to quote a paragraph you might be fond of or written particularly well.
Many good story description's I've read start with a quote and goes into a brief background of the protagonist and antagonist character-types (there a chapter on this).
As long as it provides intrigue.
Blurbs on published books are to the point and give the slightest hook- of hint at unease.
Reading more story description's will help you realise what's works and what doesn't.
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The Guide to Wattpad (& Creative Writing)
Non-FictionFrom 2013 this Wattpad Guide has offered writers like you helpful writing tips & techniques, through 60+ chapters, to raise your exposure and writing ability.