How do you find similar books?

My book is written. It's as good as it gets. It's been read, read, read, rewritten a million times, read some more and is done. It can't get any better (by me)-- it's just changes for the sake of changes.

I'm looking to start writing my query letter and synopsis. This is a newish area for me, as I usually toss my books in the "don't bother: not good enough" bin by this point.

A lot of the agents want me to include two or three authors (not long term best sellers but newish ones) that have written books similar to me. This seems fairly standard and seems to be to "prove I understand the genre". How similar do these books have to be?

Can it be completely different but share a similar tone or theme? Or does it have to have a similar arc, plot, or what?

Am I just meant to go into the book shop and read a 100 blurbs and hope I find something similar to read and then include as a tick box exercise? Or should I literally have read every single fantasy book recently published? Am I thinking way to deep into this? I just want to know what sort of books I should be referring too.

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Let's say I've just finished writing, Carrie*. What books / authors would a young aspiring Stephen King have referred to in todays market?

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*I chose this since I'm assuming everyone on here has either read it or gets the idea about it enough to suggest something. Feel free to change it.