Research isn’t always perfect. Sometimes you’ll find the perfect source or even the exact quote you need only to learn that the information you have about the source is incomplete.
It could be that you have a book or an article without a publication, maybe there’s no author name affixed or some other detail is absent. It can be frustrating, but all hope is not lost.
If you lack some or all of the information about your source, the first step is to try and obtain it. If it’s an article you may be able to search for the title to get a date or author. If it’s a book, you may either be able to search for it on Amazon or at your library. If you have the ISBN of the book, you can use an online tool to generate your bibliography from that.
Still, despite the power of the internet and your local library, there may still be times where there is missing data. If that happens, the general rule of thumb is simple: Do your best and provide the information you have.
Every major style guide has a set of rules for how to cite a source when some of the data is missing. For example, APA style has a full series of rules for just about any information you might be missing or combination thereof.
Under APA style, if the author is missing you simply start with the title of the work and alphabetize based upon that in your bibliography. Likewise, if the date is missing, you substitute n.d. (meaning no date) and do the rest of the citation as normal. If the title is missing, you simply describe the work in square brakes [Personal Letter to Mr. Smith] and otherwise identify the source as normal.
Chicago and MLA style have similar rules however, it’s always best to speak with your instructor if you’re unsure whether or not the information you have is an adequate citation.
But, while a citation may be able to survive one or even a couple of missing elements, with too much information missing it becomes impossible to know if it’s a reliable source. For example, an undated and unsigned letter could be a useful historical document or it could be a complete farce.
If your citation is missing multiple items, so many that it’s impossible to verify the work, it’s usually best to ignore that source and attempt to find one where more is known. While there are exceptions to this rule, such as many ancient historical documents, generally it’s unwise to cite a source that can’t be validated.
In the end, if you’re simply missing the title or the author of a piece, there’s little need to worry. While it’s worth the effort to try and track any missing information down, there are rules in place for correctly citing sources where such information is not available.
Follow those rules, communicate with your instructor and you should be more than fine.