I get annoyed when I'm reading a book, and there's a spelling mistake. If the author can't be bothered to get their spelling right, how reliable is his writing?
But the book I'm reading now, is full of mistakes. I'd estimate about one in every few pages. The problem the author had, is that they aren't spelling mistakes that a spell checker would find.
For example, he frequently uses the work "reign" when he means "rein" (as in horses). His worst, was "Loathed as he was to do so". There's two mistakes in just one word!
It isn't just this particular book. I'm fed up with reading books that are full of mistakes.
Using a spelling checker isn't enough. Once upon a time, publishers had staff called proofreaders, whose job it was to find and fix errors in each book before it got into print. So what happened?
I think self-publishing happened. Anyone can write a book and get it published; either as an e-book, or by paying for the print run. But this particular book was published by Saber Books, so presumably they've cut costs by eliminating the proofreader.
How can this be fixed? Actually, I think it's very simple. If you're writing a book, and getting it published via a route that omits proofreading, then get a friend (or better, three friends) to read your book and mark up the mistakes, so you can publish a clean copy.