Them = books?

Context:

In this way matters went on for some time. On a fine autumnal afternoon, Ichabod, in pensive mood, sat enthroned on the lofty stool whence he usually watched all the concerns of his little schoolroom. His scholars were all busily intent upon their books, or slyly whispering behind them with one eye kept upon the master; and a kind of buzzing stillness reigned. It was suddenly interrupted by the appearance of a Negro, mounted on the back of a ragged colt. He came clattering up to the school door with an invitation to Ichabod to attend a merrymaking to be held that evening at Mynheer Van Tassel's.