"I am reading the book."
Translation:Saya membaca buku itu.
Aku is informal and should only be used between friends and family. If you use aku with strangers it implies superiority; not something you want to do ... especially if you are a foreigner. Stick with 'saya' whenever in doubt. The same applies to using -ku as a suffix. Again, avoid this with strangers and colleagues who are not close friends.