Translation:The book is cheap.
이 if the word ends in a consonant
가 if the word ends in a vowel
In this case, if you wanted to speak generally, you would use a 는/은 topic particle, like 자동차는 비싸 - (all) cars are expensive, whereas if I said 이 자동차가 비싸, I am saying this car right here is expensive. With just 이/가 subject particles, just assume it's "the" thing.
That said, I can think of a hundred times where people used 는/은 in a sentence this simple and weren't speaking generally, but for now it's good to associate the two particles like that.