Math books are a solid investment and do not become obsolete. Looking at a digital math book is inconvenient, since learning from a math book often requires flipping between pages and the index. That is why a physical math book is more convenient. For many math topics, books from 50 or 100 years ago will also provide useful insights that are still entirely valid and true today and into the future. Books from non-math genres may not age as well, because the information may become obsolete or outdated. However, for pure math books this is not the case, which makes physical math books a good investment that could last.