I've only started reading manga about two months ago, and this is the first time I cried. Don't misunderstand me - it's not a tearjerker. I'm just really emotional. hehe ^^;;
I read volume 9 first because, from what I read in the reviews of the other volumes, the first two volumes were really long and mostly dialogue without much going on - and besides, I wasn't even sure I wanted to read this series anyways. After reading volume 9 (which I really enjoyed), I decided to read volumes 3-4.
I finished volume 10 this morning, and I loved it! I wasn't sobbing or anything, but my chest felt tight and I did cry. I really sympathized with Akinobu. It's amazing how words on a page can grab your emotions. I love that feeling.
I don't really think it's absolutely necessary to read this series in order. I mean, I never, ever read or watch a series out of order, and I wasn't even sure if I was going to like this series - but after reading Kodomo wa Tomaranai and Hanaya no Nikai de, I felt curious about how this series began.
I may just end up reading volumes 1-2 last since, from what I've gathered, this series doesn't seem connected with each other in the way that you'll miss something important if you don't read them in order. I don't know, that's just my opinion.