- I have found an error in a book/paper. What do I do?
It might happen that an answer to your question has revealed a mistake in the book or paper you are studying. It is generally good practice to notify the author about such a mistake. Many authors maintain a list of errata (errors), and the error can be corrected if a new edition of the book is published. In this way you can help other people studying the same book. If it is a serious mistake in a paper, the author might even publish an erratum.
Search first: You should always try to find, whether the author or publisher has provided a list of errata online. It is possible that the same mistake was already pointed out and maybe even corrected in a newer edition.
Be polite: It is impossible to completely avoid mistakes in long texts. Authors of books try their best to prepare good books and eliminate mistakes and misprints. Please be respectful towards them.
Use your best judgement: For books, it makes sense to notify the author even if it turns out that the mistake is in fact just a typo. For papers already published in scholarly journals, the author should be probably contacted only if there is a serious flaw in the paper.
For further reference, you may consult also this discussion at MathOverflow.