Learn Farsi: آره، این کتاب خوبه.

This is the core of spoken Tehrani Persian! In casual speech, the verb است (is) shrinks down. Because the word خوب (good) ends in a consonant (the 'b' sound), the 'is' simply becomes an '-e' sound attached to the end, written as ـه. So, خوب است becomes خوبه.

Why is the '-e' sound in خوبه written out with a ـه, unlike an ezafe which is usually invisible?

That is a very important spelling rule to remember. Even though it sounds exactly like an ezafe linking vowel, it is actually a real verb meaning 'is'. To show that a word has a reduced verb attached to it rather than just an ezafe, Farsi always writes out the spoken '-e' using a silent heh (ه or ـه).

I know بله means 'yes'. Is آره just a slang version?

Yes, آره translates best to 'yeah'. It is the standard informal way to say 'yes' in everyday conversation. You will hear it constantly in spoken Farsi, but you should still use بله in formal situations or formal writing.