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.