I have often seen posts of members trying to find a limited edition or out of print book of genealogy interest.

One way to find them in a library to to use WorldCat http://www.worldcat.org/ If you find a book here, it will tell you the names of libraries that have it, as well as what format (book, microfilm,etc). For example, WorldCat says Retonos de Espana en la Nueva Galicia (I have only found volume 2, never volume 1; and never an electronic version)) is in the following libraries:

University of California, Berkeley
University of Iowa, Iowa City
Biblioteca Nacional de Mexico
Stanford University Library, STANFORD, CA
Angelo State University Library, SAN ANGELO, TX
San Antonio Public Library, SAN ANTONIO, TX
El Colegio De Mexico, Distrito Federal
Biblioteca Nacional de España, Madrid
Princeton University Library, PRINCETON, NJ

This will list only libraries that are part of the WorldCat system. For example, the Sutro Branch of the California State Library in San Francisco also has it, but it is not shown in WorldCat. This copy circulates, which is probably not the case for the University Library copies. The Universities will have different rules about who can use their collections. For example a University of California alumni can use the UC libraries, but a member of the public must purchase a library card for $100.

Sometimes electronic copies of books are available. Do a search on google books. Some books are available as complete editions (if the book is out of print and is no longer covered by copyright). Ocassionally only a preview version is available. With luck, it may have the pages you are interested in. Google Books allows you to search for words on phrases in their editions.

The Internet Archive at http://www.archive.org/ is also a useful resource, but I've found it better for books related to American genealogy than for other countries. If you find a source here, it will be in several formats: OCR digitized (my experience is that these aren't good), black and white or color pdf files, and sometimes files for Kindle, Nook, and similar devices. Sometimes the pdfs are searchable, sometimes not.

On all of these systems you can search by title, author, keyword, and so on.

Some University libraries have special collections that have emphasis on certain regions. For example, the Bancroft Library at UC Berkeley has a lot of material on Mexico, including very rare, original manuscripts. There are special rules about using its collection, but it is available to the public at no charge (at present!).

The libraries will have different rules about what can be photocopied, who can photocopy (you or library staff), or how much of a file or book can be copied.

If the book is in a special collection, it is best to contact the library before going there; the book may actually be physically located elsewhere (especially true for the Bancroft Library).

Another way to do a search is to simply use Google. Search for the full title (or most of it). Sometimes you are lucky and you'll get a full text pdf that you can download.