Let me breakdown the different types of people who read your book, just so we are all on the same page.
Critique Partner(CP): This person often reads your book in it's early draft form, usually in portions either as you write or edit. They provide general feedback on characters, plot, setting, etc. They may also point out grammar issues, though probably not since it's still early in the process. CPs can often serve as a sounding board and help brainstorm through issues. Because this is a partnership, you are expected to return the favor.
Beta Reader(Beta): This person reads the entire manuscript, usually after you have put it through several rounds of editing on your own and through your CPs. Betas provide similar feedback to CPs, but since they are seeing the whole manuscript, they are often better able to point out overarching issues and strengths in the novel. Some authors serve as Betas for each other, but most authors use independent readers who don't expect to have their own work read in exchange.
Editor: This individual is the only one included here who is paid. They take your manuscript after you have made it as good as you can with the help of CPs and Betas and go through it line by line to help you make it the best possible novel you can. Editors will often point out major issues in plot and characters as well as sentence level issues like repetitive word choice and awkward phrasing.
Okay, so now that we are on the same page, let's talk about who you should be using.
I will say without a hint of hesitation that everyone should use CPs and Betas. In fact, I tell my authors, that I still expect them to use CPs and Betas. An agent should never, ever be the first person besides your cat to see your book.
Note: Some agents may feel differently about this and that's okay. If you see an agent posting that they don't do things this way, their way is not wrong. Everyone takes a different approach and this is a good question to ask on "the call".
It doesn't matter if this is your first book or your fiftieth. We all need help. As the author you know what your characters are supposed to say and how they are supposed to feel. We don't always get that same emotion on the page. We need other people to help us.
If you plan to go the traditional route, this is where you query (or send your manuscript to your agent). Then, depending on how hands-on your agent is, they will possibly do another deep edit. You can never have too many eyes looking for mistakes in your manuscript. Your agent will want to make sure you are submitting the best possible work you can.
I advise against hiring an editor if you plan to go traditional. For one, one of the bonuses of going traditional is the fantastic in-house editing your book will receive at no cost to you. No sense in paying for a service that will eventually be free.
The second reason is less obvious and has to do with how you look as an author. You hire a professional editor to get an agent, sign with them, and sell your book. So when you write the next book, are you going to hire an editor again? Probably not, since you already got your agent. So then you send the new manuscript off to your agent who wonders why this book is not as polished as the first. You've unknowingly set yourself up to look bad. No one wants to look bad.
Now, let's say you aren't going the traditional route and you plan to self-publish. You need to hire an editor. I'll write another post on why another day, but for now, just trust me on this. You need to hire an editor, who you pay, with actual money.
Now, I get it. Editors, or the good ones, don't come cheap. I personally saved up money and tucked it aside so I could hire the editor I wanted. I also paid for a proofreader, but you get the point. If you expect people to pony up their hard earned money to buy your book, you need to do the same to get it ready.
Maybe that means you don't get to publish as soon as you wanted. Maybe it means self-publishing isn't really an option for you. Only you know your situation. But if you decide to publish without hiring an editor, don't be surprised if the reviews are lack-luster and sales follow suit.
So that's it. To help out any new writers reading today, how about we fill the comments with tips on finding CPs, Betas and Editors. Feel free to leave any links to CP match sites that you know of.