As nouns the difference between textbook and monograph is that textbook is a coursebook, a formal manual of instruction in a specific subject, especially one for use in schools or colleges while monograph is a scholarly book or a treatise on a single subject or a group of related subjects, usually written by one person.
As an adjective textbook is of or pertaining to textbooks or their style, especially in being dry and pedagogical; textbooky, textbooklike.
As a verb monograph is to write a monograph on (a subject).