Past participle of leer.

Hola Allen Z.

You're absolutely right that the standard past participle of leer is leído.

However, in the example you mention, where you see leídas, the participle is being used as an adjective and therefore must agree in gender and number with the noun it refers to, in this case, a feminine plural noun like novelas.

This happens in constructions such as lleva leídas tres novelas, where leídas agrees with novelas. When the past participle is used with auxiliary verbs like haber (e.g. he leído), it always stays in its default form and doesn’t change. But when it’s used to describe the result or progress of an action and directly modifies a noun, often after verbs like estar, tener, or llevar, then it behaves like an adjective and must match the noun in gender and number (e.g. novelas leídas, artículos leídos).

So, in short: leído is correct on its own, and leídas is also correct when it refers to feminine plural nouns and acts adjectivally.

Hope that clears it up!

Feliz semana

Silvia