The Library of Congress Online Catalog contains many titles on genealogy, local histories, and family histories that the Library of Congress has not yet digitized. However, you might be able to find some of those books in digital form elsewhere.
Let's start with an example of a family history in the Library of Congress collections:
Here are some resources for online copies of books that you can try:
Founded in 2008, HathiTrust is a not-for-profit collaborative of academic and research libraries preserving 17+ million digitized items. HathiTrust offers reading access to the fullest extent allowable by U.S. copyright law. Usually when an item in our collections has been digitized and made available on Hathitrust, our catalog record will note that, and include the HathiTrust link to the digitized item. In this case, the catalog record does not include the HathiTrust link (probably because it is from the Library of Congress old card catalog).
The Internet Archive, like Hathitrust, is another non-profit library of millions of free books, movies, software, music, websites, and more.
FamilySearch is a genealogy subscription database. They require that you create an account with an e-mail address, but once you do that, the information is free. FamilySearch is digitizing more and more family and local histories. So are some of the other popular genealogy databases. If you already have a subscription to some other genealogy database, check to see if they might have digitized the book.
Many family histories and local histories still have not been digitized. If you cannot find a digitized version at one of the sources above, you may want to see if you can locate another library that has a print copy of the book. The Library of Congress does not circulate its family or local histories via Inter-Library Loan, but public or university libraries often do.
Worldcat aggregates the online catalogs of a great many libraries. At their landing page, you can enter the title of the book you want, and your zip code. It will display a list of libraries that have the book, in order by proximity to that zip code.
These are people who specialize in requesting and duplicating materials for those who live out of town.