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I dunno... when selecting books/doujin it's like clothings.

*Sayo's Kyourin Works is good but it doesn't really come in the typical illustration book style as large single page illustrations that covers the entire page. It's more like 1-3 illustrations per page.

I can list circles and artist i've purchased already ^^. That might give u a head start.

I'd recommend Moto4% Illustration book if you don't have it.
- http://ekizo.mandarake.co.jp/shop/en/item_s-82459.html
*Same price as Amazon.jp (about >.< . s/h probably cheaper at mandarake)

Mitsumi Misato / Cut a Dash!
Tatsuki Amaduyu / Blazer One
Miki Miyashita / Kyogetsutei
Carnelian
Suzuri / KeroQkei
Motoyone / Tennenseki
(couldn't find Sca-Ji's works, since motoyone, suzuri and sca-ji's stuff are all in the KeroQkei circle)
Akane Ikegami / AMR
Mutsumi Sasaki / Nekoyashiki

That's all I can think of right now :)