Nobody is perfect. And self-publishers without editing staff are especially prone to typos and other mistakes in the final product. But even well-known authors with publishing houses and armies of proofreaders behind them churn out books with plenty of errors, at times. Like the wrong character uttering dialogue, or the wrong name popping up for a character, or words in incorrect order. Spellcheck is no substitute for a sharp eye and a fresh mind.
But minor errors can be corrected and uploaded (or dismissed and ignored), especially for e-books. On the other hand, major alterations require a whole edition change complete with new ISBN. This is an opportunity for a fresh cover to go along with the rewrites. And some authors look at such changes as a means of boosting sales of an older book.
So, if doubts about a few passages have crept in, or new information has triggered a re-think of parts of an existing story, this can be a chance not only to correct spelling errors but to use creativity in enhancing a good tale with additional paragraphs or chapters, updated artwork, and perhaps even a new title.
If you write, have you published any second (or third) editions?
If a new edition comes out for a book you’ve read, do you buy a copy or pick it up at a library to check it out?