Publishers often go through their book catalog to see what books are doing well and which books are not meeting their quotas. For books that are unpopular or costing a publisher money they may find themselves on the "out of print" list. This means that the publisher is no longer making copies of the book. If you are looking for a book and can't find it, there are a number of ways to check to see if the book is still in print.
Type the title of the book or the ISBN number into a website such as Amazon, Barnes and Noble or Borders to see if the book is available. If the book is out of print the website will indicate this.
Contact the publisher with the name of the book and author. Inquire if the book is available and still in print. You can also contact the author through email via their website to see if they know if the book is available. Try social networks like Facebook or Twitter if you cannot find their email address.
Visit your local bookstore. If the book is still in print the store will be able to order it. If it's not they will have further information about the status or ways that it may be purchased, even if it is no longer being printed.