El libro está abierto.

• Ser is used for permanent characteristics or inherent traits, while estar is used for conditions or states that can change.
• Being "open" is considered a temporary or changeable state, so estar is appropriate here.

• In Spanish, adjectives must agree with the noun they describe in gender and number.
• Libro is a masculine noun, so the adjective takes the masculine form abierto (rather than abierta).

• Abierto is originally the past participle of abrir ("to open").
• In this sentence, it also functions as an adjective describing the state of the libro.

• Sometimes you might see El libro se encuentra abierto, which has a similar meaning but sounds more formal.
• However, está is more common in everyday speech to describe states or conditions.

• Está abierto describes a current state: the book is in an open position right now.
• Está abriendo describes an ongoing action: someone is in the process of opening it at that moment.