If I found a source in a book at a library I would cite the source, information about the book, and the repository, library,branch, city, State, and country, but if I found the book online I would still cite the same information about the book but instead of a repository I would add the url needed for others to find the same book. If it is found in the library of congress then I would add the url for the online source or the location for an in person visit but the actual source is the publication you are citing, where you found it is not something different but just part of the citation.