The main classes have one single capital letter. The sub classes have either one, two, or sometimes even three capital letters. Arab integers from 1-9999 are used for further subdivisions. Decimal extensions can be used in case more space is needed. The final part is the Cutter number.
Cutter numbers are for individual books.
In this case B stands for Blaire, who is the author of this particular book. A year can be added at the end of the number. Here 1990 is the year of publication. The Library of Congress Classification was originally created by more than one person. This is the reason that some of the classes look a bit different than others.