The lure of a royal ancestry can be overwhelming. For those living in the United States, being "related" is prestigious and helps us to think we are something special. In other countries, with real royalty, being related definitely increases your position in a class conscious society. But what is the reality? Obviously, kings and queens had children and therefore descendants but what makes you think you are one of those descendants?
Unfortunately for family historians, there was a definite period in professional family research, when there was a tendency to provide what the client wanted, regardless of the facts. Trend culminated in the late 1800s and early 1900s in the United States but had a longer history in Europe. Many of the family histories (surname books) published during this time have few, if any, sources listed and are really pure fantasy. For an extended discussion of this topic see the following:
Weil, François. Family Trees: A History of Genealogy in America, 2013.
But the crucial question with most pedigrees that go back to royalty is the validity of the research connecting the living person back to the royal ancestor. Depending on your own ancestral lineage, there are some mostly reliable books tracing Americans back to royalty. One of these is the following:
Weis, Frederick Lewis, Walter Lee Sheppard, William Ryland Beall, and Kaleen E Beall. Ancestral Roots of Certain American Colonists Who Came to America before 1700: Lineages from Alfred the Great, Charlemagne, Malcolm of Scotland, Robert the Strong, and Other Historical Individuals. Baltimore, Md.: Genealogical Pub. Co., 2004.
By combining the family history submissions of thousands (perhaps millions) of people, the FamilySearch.org Family Tree sometimes has surprises when it comes to connecting your family to royal lines. But unless every ancestor between you and the royalty has been verified, you cannot safely claim that royal ancestry. FamilySearch.org does have two major sources for information about the royalty in Europe:
For many years the FamilySearch Community Trees database was the go to place to find current information on the royal houses of Europe. In 2015, the databases were incorporated into the main Genealogies section of FamilySearch.org. Here is a screenshot showing where the website ended up:
The Community Trees section is now searchable from the main menu. Unfortunately, the discrete identity of the individual databases has now been obscured. Searching by the name of your ancestor is not necessarily going to let you see the connection. This is the loss of a great resource.
Another avenue of approach is the FamilySearch.org Catalog. If you search on "royalty" as a keyword, the program will return over 360 books and other records about royalty. The other major databases also have substantial resources about royalty. The main difficulty of doing any research where there is an assumption of royalty is separating fact from fiction. This is where it is very important to do careful, systematic and source supported research and try to dampen the enthusiasm of the hunt, when you begin to find that your relatives have been identified as the descendants of royalty. This is particularly true when the records (usually books) start claiming a relationship through an illegitimate child of royalty when there are no substantiating sources. Caution is advised.