I've just finished a novel and I've read that at this point I should be sending it to critique partners and beta readers for feedback. I don't know anyone to do this, and I am a very thorough editor, so feel like the book is as polished as it's going to get. Is this feedback period really crucial to making my book a success? And if it is, where do I find these readers?
Personally I believe this is probably the most important step in getting your book ready. As the author, you know everything about your story and your characters so are unlikely to see gaps in logic or places where information might be missing. Getting fresh eyes on the book is crucial for you to find out if everything is working the way you intended it to.
Critique partners and beta readers also fulfill different purposes. A critique partner might read the book chapter by chapter as you write it and offer feedback as you go. Or they might read it when you have finished a first or second full draft. Their notes will include grammar and syntax errors, punctuation and suggestions about plotting, pacing and character development. Finding other writers to be critique partners is usually a good idea because they will understand these things and be able to offer the right kind of feedback at this stage.
Beta readers are readers who will read the full manuscript once you have completed the revisions your notes from critique partners threw up. These readers won't give you the detailed notes your CPs did, but will be able to identify how the book reads as a whole. They will let you know if they believe and empathize with the characters, if the plot is engaging and if they feel the pacing works. They will also let you know if the book is an enjoyable read, and may be able to tell you other books they've read that are similar.
In terms of finding people to act as CPs and beta readers, there are numerous options. Join a writing group, in person if there is one in your local area, or online if there isn't (there are some very active critique groups at writing.com). Engage with other writers via their social media and blogs. Whatever stage in your writing career you are at, there will be other writers out there at the same stage and many will be willing do a MS exchange. Engage with readers who talk about books online and ask them to beta for you. You won't always get a yes, but a lot of people are excited to read new books before they are published and will jump on the chance.
Good luck with the book!