It really depends on your skill with cards. Most people go through all CC volumes, i skipped the first one and just finished the third. Need to get through two more.
I say vol 2 is the best even though i haven't read 4 or 5 simply because vol 2 has a huge section on the theory of magic and presentation. Giobbi did a fantastic job of exploring each step in detail.
Each book also contains some awesome tricks that i use day in day out.