Ask nicely in a well-composed, personal email. Don’t send your request by Facebook or Twitter -- email is best, when possible. Here are the important points to include in your letter when asking for a book quote:
• Quickly introduce yourself in a personal way, then state that you’re asking for a book jacket quote.
• Briefly offer the details about your book: genre, story, publisher, release date.
• Explain why you think the contributor is a kindred spirit who might like your book. Possible reasons could be: you also like his/her writing; one of the author’s titles compares well with yours; or you both love and write about the same topics.
• Give the contributor a deadline for the potential endorsement, but offer as big a timeline as possible.
• Let the contributor know where the blurb will be used. Will it be on your book cover, your author website, or both?
•Don’t forget to say thank you!