If you’re a small or self-publishers just starting out, some helpful definitions and links can be found here:

The International Standard Book Number (ISBN) system is administered by the ISBN Agency, which assigns publisher’s prefix numbers that are unique to each company. The ISBN number consists of thirteen digits, which are separated into four parts. There is a fee to apply ISBN numbers, and it normally takes 10 business days or more to process an application. The ISBN Agency has other helpful information on their site and an electronic application form for you to use. The form can also be downloaded for non-electronic submission.
Please note: when you use one of your ISBN numbers you must report the assignment to R.R. Bowker to be eligible for a free listing in various directories. This will assist people in finding your company in order to purchase the title. To register your title with R.R. Bowker's Books In Print, go to

Library of Congress Control Number
For more information visit the Library of Congress web site.