Juan Ruiz, a medieval Spanish poet also known as the Arcipreste de Hita, is best known for his poem Libro de buen amor or The Book of Good Love. Several English editions are available.
There are several techniques you can use to search for copies. First, look for free etexts by searching the title in Google. Next, a search of Google Books will produce several texts, available in partial facsimile.
If you wish to find library copies, and you are a US university student, your library catalogue should have a portal to WorldCat, which is a uniob catalogue of US libraries. In worldCat, you can search library holdings, to find whether the book is available in your school library, other libraries nearby, or is available through InterLibrary Loan, In Canada, the Amicus database is the standard union catalogue; Copac (copac.ac.uk) has a similar finction in Britain.
To buy a copy of the book, you should look at aggregator sites such as Bookfinder or ABE books.