I am interested in learning design patterns and would like to know what are considered top tier books in learning this subject.

Is there a book out there that's the de-facto standard for describing best practices, design methodologies, and other helpful information on design patterns? What about that book makes it special?

Design Patterns: Elements of Reusable Object-Oriented Software comes very close to my definition of a canonical book on design patterns. According to its wikipedia article (emphasis mine): 


  The original publication date of the book was October 21, 1994 with a 1995 copyright, and as of July 2010, the book was in its 38th printing. The book was first made available to the public at OOPSLA meeting held in Portland, Oregon, in October 1994. It has been highly influential to the field of software engineering and is regarded as an important source for object-oriented design theory and practice. More than 500,000 copies have been sold in English and in 13 other languages.


Ward Cunningham, a design patterns pioneer, maintains an online catalog of the book's patterns on WikiWikiWeb. And according to the Wikipedia article on design pattern (again, emphasis mine):


  Design patterns gained popularity in computer science after the book Design Patterns: Elements of Reusable Object-Oriented Software was published in 1994 by the so-called "Gang of Four" (Gamma et al.). 


There are quite a few other books referenced in the same article  as notable in the genre:


  
  Pattern-Oriented Software Architecture Volume 1: A System of Patterns, by Douglas Schmidt, Michael Stal, Hans Rohnert, and Frank Buschmann,
  Patterns of Enterprise Application Architecture by Martin Fowler, 
  Enterprise Integration Patterns: Designing, Building, and Deploying Messaging Solutions., by Hohpe, Gregor and Bobby Woolf, and 
  Head First Design Patterns, by Eric T. Freeman, Elisabeth Robson, Bert Bates, and Kathy Sierra.
  


Of those I've read Fowler's book, it's highly influential and a good read. At certain points it's a little vague for my taste, but overall it's an enjoyable book. There's an online catalog of the patterns included in the book, with minimal descriptions.

I've also  skimmed through Head First Design Patterns, and if you have read any other book of the Head First series, it's of the same high quality and as enjoyable as most books in the series: 


  Head First is a series of introductory instructional books to many topics, published by O'Reilly Media. It stresses an unorthodox, visually intensive, reader-involving combination of puzzles, jokes, nonstandard design and layout, and an engaging, conversational style to immerse the reader in a given topic.


The term "design pattern" is somewhat vague, as every general reusable solution can be considered a design pattern. I've always noticed a tendency to apply the label on the solutions described in one of the notable books I've listed above, and more specifically the Gang of Four and Fowler books. Design patterns do not follow a unique development process, they are normal software solutions that happen to be immensely reusable and they are extremely hard to identify. 

But if you compare the online catalogs for both books with the contents of language specific books you'll notice that they are often used as templates. So I'd say that both books are very close to being canonical references, with the GoF book being the more important one from a historical perspective, even though both books are limited to object oriented programming. 
