GridCtrl master, details;

master.AddIndex(ID);
master.AddColumn(LIBRARY_NAME, "Library name");
master.WhenRowChange = THISBACK(UpdateDetails);

details.AddColumn(BOOK_NAME, "Book name");

void UpdateDetails()
{
    SQL * Select(ID, BOOK_NAME).From(BOOKS).Where(LIBRARY_ID == master(ID)); //master(ID) returns ID from currently selected row
    while(SQL.Fetch)
        details.Add(SQL[BOOK_NAME]);
}