original answer still stand for me, that it is "not" to late to register my book? Here's what they wrote, and please keep in mind I asked them the same question as you: "Mandatory deposit (17 U.S.C. section 407) requires the owner of copyright or of the exclusive right of distribution to deposit in the U.S. Copyright Office for the use of the Library of Congress two complete copies of the best edition within 3 months after a work is published. Copies of all works under copyright protection that have been published or distributed in the United States must be deposited with the Copyright Office within 3 months of the date of first publication. Electing not to register your copyright in the work with the Copyright Office does not exempt you from the mandatory deposit provision of the law." Like the Certs twins, both answers are right. Registration with the Copyright Office (there is no "registration" with the Library of Congress) requires that you deposit two copies of your book, which, in turn, satisfies the Library of Congress' deposit requirement. In other words, you don’t have to worry about the Library of Congress if you register your work with the Copyright Office.
What's up with the Library of Congress? The creators of the Library of Congress had this dream that they would build a Noah's Ark of America's creative efforts -- containing two copies of every published work. Laws were enacted demanding that artists, writers, and musicians deposit copies of their work within three months of publication. If they failed to do so, the Library was entitled to demand copies and to mete out financial penalties for those who disregarded the requests. In practice, we've never witnessed an LoC request or penalty, and our research shows that the chances of it happening are lower than being struck by lightning this year (Bonus lightning fact: People on cell phones are struck the most!)