Is the word bravo acceptable in English books?

Answer 1

Bravo is acceptable use in the English language:
1. bravo: exclaim.
used to express approval for a performer.
(From The Concise Oxford English Dictionary)

Answer 2

Fusion of Latin pravus and barbarus. French and English brave derived from it.

Sometimes the Italian female form brava is used for a woman, and the Italian plural forms bravi and brave used for a group of men (or men and women) and a group containing only women, respectively.

It is held by some philologists that as "Bravo!" is an exclamation its form should not change, but remain bravo under all circumstances. Nevertheless "bravo" is usually applied to a male, "brava" to a female artist, and "bravi" to two or more."

Earlier it was used as a noun meaning "desperado, hired killer" (1597). Superlative form is bravissimo.