Queries & Synopses: Boon, Bane, Both?
Both. Mostly boon, though, because--as Jeffe mentioned on Sunday--both types of summaries force you to crystalize the essence of your book. If you can't do it, there's a really strong chance your plot isn't as clear nor as tight as it should be. You may have written 90k of what felt like a compelling story, but if you can't distill it down to 250(ish) words, then open a blank page and chapter-by-chapter write a one-sentence summary for what happens in that chapter. You should be able to condense those 30(ish) sentences into a shorter summary that still tells a story. The continual refining of your "short story" is akin to zooming out, just keep mentally hitting the Ctrl- keys until you've hit the requested length. If you can't craft a flowing story from the chapter summaries, then you might have embraced an author's nemesis--the tangent. Fun to write, but nothing that advances the plot or the character development. Thar be edits in your future, matey. Better to know that before you send your "completed" mss into public.
I consider synopses and the meat of queries to be a critical "is my book ready for submission/public" check.
Also, as James said, you've got to be able to write your own marketing copy. From your back-cover blurb to the hook on your website to your social media promos. Regardless of which publishing path you've taken, those super short BUY ME statements are necessary. Here are examples for my Immortal Spy UF Series.
Side tip: When you're promoting your book on social media, in addition to the short hook, use your genre hashtags, include a Call To Action (Pre-Order! Buy Now!) with the corresponding link, an image that includes the cover art and book title. Use the title in promo the text too. Make it easy for a total stranger to ONE click-to-buy and ONE click-to-share.
#NewRelease: THE EXPOSED SPY #UrbanFantasy. Whistleblowers found a fatal flaw in the Mid World defense system. Can Bix & the Berserkers save them and the project before the government kills them all?— K. A. Krantz (@KAKrantz) October 13, 2020
Other Retailers: https://t.co/3ZsMPfS2kI pic.twitter.com/ZsAXq52Y1t
It breaks my heart when I see book promos that are little more than "I have a new book out today!" Without the supporting info mentioned previously, you're disinviting potential new readers to discover your work. It's like saying, "If you don't know the details without me telling you, then you're clearly not cool enough to hang with me." Eeep. That's like, anti-marketing. The un-sale notice. Don't, don't do that.