This is a typical entry in the master_library.cfg file.
As this file cannot be changed via eScript programmes, this is a semi-permanent file. It can be edited using a text editor. That is the only way to remove a book or books from the file. The file can be deleted in the event you want to recreate your Master Library with all new books.
Please note: Unless you understand the format of config files you should not try deleting individual books. The best option is to delete the file and start with a fresh empty file and add books using the addsysbook command.

book 3
{
    pages    4
    title    Regarding Llamas
    author    Simon Sammartine
    p1l1    Llamas are curious
    p1l2    beasts, shaggy and sought
    p1l3    after for their wool, yet
    p1l4    of a curiously arrogant
    p1l5    disposition reflected in
    p1l6    their eyes. They live in
    p1l7    mountainous areas, though
    p1l8    who may have first
    p2l1    tamed them is lost in
    p2l2    the mists of history.
    p2l3   
    p2l4    ‘Tis a well-known fact
    p2l5    that llamas can indeed be
    p2l6    tamed, and used as
    p2l7    grazing animals, for their
    p2l8    meat, and of course for
    p3l1    their wool. Yet ’tis
    p3l2    lesser known that their
    p3l3    ornery disposition and
    p3l4    tendency to spit at those
    p3l5    they dislike makes them
    p3l6    appealing guard creatures
    p3l7    as well, though they have
    p3l8    little sound with which to
    p4l1    sound an alarum.
}