Make sure you buy vols. 5 & 6 right away (well worth it). Volume 4 has one heck of cliff hanger.
Volume 6 is the best volume by far IMO and it closes the second arc quite nicely.
Yup yup i actually already bought Vol. 5 a few weeks back.
(I cheated and read Vol. 1-4 online beforehand and yes go figure the site i was reading on ended at Vol. 4, so they got me with that cliff hanger!)
I would have picked up Vol. 6 also but unfortunately it was out of stock at the time so i do still need to get it.