Humans are endlessly fallible. We are never going to be error-free, all the time, no matter how careful we are. Some mistakes in stories, essays, books, etc. are because a writer (and editor and proofreader, if they’re involved in the process) doesn’t know any better. For instance, all the times I’ve read “The data is …”, I know the writer simply doesn’t know that data is a plural noun and that it really should be “The data are …” (And if there’s an editor or proofreader involved in those cases, I’m embarrassed for them for not knowing this.) Other mistakes are because the brain corrects simple errors, such as when we type reed when we meant read. I recently sent an e-mail to someone that I had reread at least three, maybe four times, and still I didn’t notice that I had typed breasts stroke.
Even if small, inconsequential errors get through, careful proofreading should catch the larger, embarrassing mistakes. For example, earlier this year I read a book by a well-known writer that has two major characters. The narrative kept shifting between the characters, told from their alternating points of view. Half a dozen times, when in Character A’s point of view, she was identified as Character B. I don’t blame the author for this mistake. After being immersed in a book for a couple of years, with all the revising, rewriting, and rereading that that entails, a slip like that is easy for an author to miss. And maybe I can give the editor a pass; she too might have read the book too many times to notice. But the proofreader absolutely should have caught that. (Of course, there is another possibility—the proofreader flagged the errors and the printer neglected to make the changes, but I’m going to stick with proofreading right now.)
Proofreading can be necessary at different times in the production of a book. An author might want her manuscript proofread before she starts querying agents. Although I agree that you want to send a high-quality product to an agent, the need for proofreading is not vital at that stage, especially if money is a concern. It can cost several hundred dollars to proofread a 90,000 word manuscript, for example, and chances are—provided you and your editor have gone over the manuscript carefully—any mistakes a proofreader catches would not reach the level of a make-or-break deal with an agent.
If the agent has found too many simple mistakes, she might suggest proofreading before sending the manuscript to editors. At that point, it’s worth it for the author to spend the money if she doesn’t trust herself to do a good job proofreading. After that, once a book is bought by a publishing house, proofreading is in the hands of the publisher.
If, however, you are self-publishing, proofreading remains your responsibility. And it is a very important one. The steps with a self-publisher are that, first, you send in your manuscript and the publisher typesets it (to use an old-fashioned word for a digital process). Second, the publisher sends the typeset book interior to you for you to okay for publication. At both of these stages, the book should be proofread. The latter is your last chance to make changes, but the time before that is the more important one. You want the self-publisher typesetting a manuscript that is as pluperfect as you can make it. Once a book is set, making changes is time consuming, and it runs the risk of new errors being unintentionally introduced.
When clients hire me to proofread books they are self-publishing, I recommend they use me at one of these times, not both. If I proofread a book twice in a short period of time, I might miss things. On the other hand, I recently was asked to proofread final proofs of a book I had edited a couple of months earlier. The self-publisher had recommended to the author that I proofread the manuscript before it went to the printer; the author refused, opting to do the proofing himself. As I read the proofs, I found numerous errors. Some of them were the result of the author having made changes to the edited manuscript that I never saw; some of them were mistakes I had made in the editing (such as neglecting to add a comma after an introductory phrase); and some of them were typos that got past everyone in the original stages. It would have been far better if I had proofread the manuscript before it went to the printer.
On the other hand, with another book I had worked on both as developmental editor and line editor, I opted to proofread after the book was typeset. The author had done an excellent job proofreading before it went to the self-publisher, and I found only simple, easily corrected errors.
The bottom line? Proofread, and then proofread again. And don’t do it all yourself. Let someone else do at least one round of proofreading for you. If possible, have that person be well versed in grammar and rules of punctuation. If you’re afraid neither you nor anyone you know could provide a high level of proofreading, hire someone to do it. You don’t want to spend years writing a book, and then spend good money on getting it self-published, only to have readers encounter so many mistakes, they never finish reading your book and—worse—recommend to their friends that they not bother buying it either.