Every book will be assigned an ISBN and the back cover will include a barcode that can be scanned for stock control with all major online retailers. An LCCN is an additional identifier required for a book if you wish it to be considered to be stocked as libary reference material. It is not relevant for other material like fiction.
The inclusion of an LCCN does not mean a book will be stocked into the Library of Congress. In our experience, self-published titles are actually rarely accepted by the libraries. This is in no way a reflection of how a particular book might sell, just that the libary have more of a quality guarantee if they typically limit acceptances to top named publishing houses as it is easier to verify the material.
If you have written a reference book, you are based in the US and you would like to obtain an LCCN then you will need to apply for this. Applications are typically completed within 3 weeks. We regret we are not able to provide an LCCN for books published under our imprint at present.