treatise vs monograph

treatise and monograph both are nouns.

Word	Noun	Adjective	Verb	Adverb
treatise	Yes	No	No	No
monograph	Yes	No	No	No

As nouns, monograph is a hyponym of treatise; that is, monograph is a word with a more specific, narrower meaning than treatise:

Other hyponyms of treatise include dissertation, thesis, pamphlet, tract.

treatise (noun)	monograph (noun)
a formal exposition	a detailed and documented treatise on a particular subject