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The answer is this: It is entirely up to you.
If you've taken your book as far as you can take it—and either you're stuck, or you just need a fresh pair of eyes--then you might consider finding an editor.
Taking your book as far as you can take it means different things to different authors.
It could mean you've written a first chapter and an outline and you want feedback before you go any further. Or it can mean you've written several drafts of an entire book or proposal. Only you can say if you've taken it as far as you can go for now.