You've sweated the hard work over weeks and months (maybe years) of writing a book, website copy, or classroom syllabus. Don't release or publish your work until a skilled, experienced editor has reviewed it and you've revised it based on professional recommendations. I'm a professional editor, and a darned good one if you believe my clients' recommendations, but I have always relied on that last set of eagle-eyes to alert me to what I missed when caught up in my own creative process.
There's far more to "editing" than proofreading. More on that in another post, another day.