There are numerous ways in which you can find books and articles regarding famous members of royal families. Having a family tree to work from certainly helps identify which family members you might want to look for and there are numbers of family trees or databases on the Internet. One is Hull University’s” Directory of Royal Genealogical Data: http://www3.dcs.hull.ac.uk/genealogy/royal/http://www3.dcs.hull.ac.uk/genealogy/royal/It lets you find English monarchs and Identify their children, grandchildren, and other family members.
I’m sure you already know you can search for information on a particular royal family member in Wikipedia. The article itself may be a good start but more importantly most articles have a source list at the bottom of the page. Follow up on the list for books and other published materials getting the title, author, publisher and date. Google books and Internet Archives http://www.archive.org/http://www.archive.org/are two of the largest sources of digitized books and both have decent search engines. While Google Books contains materials in the public domain (out of copyright), when you locate a title they don’t have digitized, use the ‘Find in a Library’ link. This does two things. It links you to WorldCat and (9 times out of 10) identifies which libraries near you hold a particular title. You could also just search WorldCat directly. Either way, once you have the information on the book use your library card to request a book (or books) through the ILL(Inter-Library Loan) desk at a local library or fill out a request online. Where I live it takes 2-3 weeks to get the book and the normal cost is $3 per book. Occasionally with rare or valuable books the lending library will charge an additional fee ($10-25).
In any books or articles I try to always read footnotes, endnotes, and bibliographies, for information on other books or articles the author used in his or her research. Not every book has notes or a bibliography but most do. Following through and doing this in turn on the referenced materials often leads you to the most authoritative works on a subject.
Then you may or may not know about JSTOR. It's a subscription source of over a million academic journal and scholarly articles that includes rich periodicals like The English Historical Review, Spectrum, Journal of British Studies, The Scottish Historical Review, etc. If you belong to a local library you probably already have a JSTOR subscription through them allowing you online access (usually indicated by having a 14-digit library card number). While these resources are certainly not the only ones available to you they basically serve as good examples of how to find books and journals. Using the ones I mentioned above I can usually locate a large percentage of books and articles I look for on medieval subjects. Hope this helps.