When a work is important, it means that it has had a significant impact on its field and/or other fields. Some works get published and do not seem to have any impact on others. Other works may be cited hundreds or thousands of times, or may be discussed and debated in journals for decades. There is no one way to tell whether a book or article is important. While it is to some extent relative, here are some things to look for:
For a book:
Look for reviews in scholarly publications. Try searching for the title of the book in OneSearch or library databases that are important to research in your field (for example, JSTOR for the social sciences and humanities, ScienceDirect for the sciences), to see if there are reviews of the work. The existence of reviews is a good sign, but read them and check for relevance.
Find out how many times the book has been cited. You can use Google Scholar to do this. If something has been cited a significant number of times, that shows that it is important in the field.
How many times is significant? Again, it depends. You need to look at the age or copyright of the book and compare it to other works on the subject. A newer book obviously will not have as many citations because it takes time for scholars to read and use a source and for their subsequent works to be published. Comparing it to works on a similar topic with a similar copyright will give you a clearer view.
Look at the publisher. Scholarly publishers like University presses are more selective about the works they publish. However, just because a work is of high quality does not mean it necessarily has had an impact on its field.
For an article:
Find out how many times the article has been cited. You can use Google Scholar or the Web of Science database (sort by “Times cited--highest to lowest”) to achieve this. If something has been cited a significant number of times, that shows that it is important in the field. How many times is significant? Again, it depends. You need to look at the publication date of the article and compare it to other works on the subject. A newer article obviously will not have as many citations because it takes time for scholars to read and use a source and for their subsequent works to be published. Also, a topic that does not have a lot of scholars writing about it will not have as many scholars citing it. Comparing it to works on a similar topic and of a similar age will give you a clearer view.
Look at the journal. Is the article published in one of the important journals in the field? Usually, the best ranked journals in the field will publish the highest quality works from the best scholars in the field. However, just because something is of high quality does not mean it has made an impact on the field, so the quality of the journal should not be the only criteria you use. For more information on evaluating journals, take a look at Determining Whether a Journal is Significant in my Field.