Shouldn't "magni" be "magnae" as it is modifying "poetae"?
Fīliae vestrae dē libris magnī poētae saepe cogitābant.
The quote is from Wheelock's Latin, chapter 6.
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There is agreement, in fact! Both of these words are masculine genitive singular.
The trick is that poēta is a masculine noun, despite being in the first declension. So the genitive singular is -ae, just like puellae. But agreement depends on gender, not declension, so it takes the masculine -ī, just like puerī.