Should I Get a Book Editor?
I haven't quite figured out why people ask this question so much. Unless you think you're some kind of literary demi-god, there is no reason in the world why you shouldn't put at least minimal effort into editing your manuscript once you've completed it. Editing services are an integral part of any publishing process, but it's especially important for book publication. If you have followed (what I believe to be) the most simple and effective timeline for writing, you just vomited (tens of) thousands of words out. There's a slim chance in Hell that they're going to be grammatically correct, rationally organized, or arranged in a way to be as compelling and clear as possible to a reader. Now that you've finally finished your manuscript (yay!), request a quote for editing services so you can proceed to the next step in the publishing process.
Can't I Edit My Own Book?
Yes and no.
I would encourage you to review and revise your book even before sending it to a book editor. However, I would still push you to send it to at least one book editor. Reviewing and revising your book helps you solidify your own vision for what you want the book to look like. Imagine you want to build a custom home. You first need a mental picture of what you want the house to look like, the architectural style, colors, even how many bedrooms, closets, bathrooms, and staircases are all part of this initial vision. Then you might sit down and list out the specifications of the house. Then comes drafting a general idea of what you want the house to look like, either with pen and paper or basic rendering software. If you do this much work and put this much thought into your ideas, when you hand them off to an architect, they are going to have a much easier time giving you what you were looking for. They will only be editing your concept for the sake of building codes, budget, current technological limits, and the like. But if you just walk up to the architect and say "I want a big house," you're going to get a house (book) that is more architect (editor) than homeowner (author).
However, it's not helpful for you to spend a drawn out, indefinite amount of time revising the work on your own. I recommend reviewing then revising your manuscript no more than twice. Reviewing your manuscript means simply that you read it. You can note changes that you want to make, but it's helpful to have the discipline not to give in to the urge to make those changes while you're reading. Revising your manuscript is a separate process that involves only implementing the changes that you noted. Keeping these two processes separate strengthens your self-editing efforts. This is why printing the manuscript out and marking it up with ink and highlighters can be more beneficial that reading digitally. Reading the manuscript as a digital document can make it too easy for your hands to reach for the keyboard to do "a quick edit" here and there. The next thing you know, the review process is completely abandoned and you've prematurely begun making revisions. I find it most helpful to just let yourself experience the book, as a reader would, without trying to think too much like an author.
However, trying not to think like an author, especially when reviewing your own work, is something that takes a tremendous amount of effort. Even when we try our hardest, there will be things that we miss—guaranteed! This is why it's so important to hand the manuscript off to a professional once you've completed your own revisions.
So, yes, you can edit your own book. But you will still benefit from having someone else edit it who was not the creator of it. They will have the distance from the work that's needed to gain a helpful perspective on what may need to be changed.