The movie was a heartbreaking masterpiece. Easily the equal of Ghibli's work. (You can rent it on Google Play, which sometimes runs a 99cent 2-day movie rental.)
It looks like the numbering of the chapters are different compared to the tankoubon.
For example, chapter 6.1 is in fact chapter 9 of vol. 2.
Vol 3 is out not long ago too (dunno if it's the last one but it has up to chapter 27).
AMV for those interested, might contain SPOILER! Click at your own risk, you've been warned... The song is really good though... さよならの朝に約束の花をかざろう <-- Sayoasa