looks like this art book is available through pre-order only and it's over.
http://shop.kodansha.jp/bc/yoyaku/comics/mushishi/
If anybody is really keen on getting it, try Yahoo! Japan auctions. The original price is 4800Yen. Saw an auction close at 6800 Yen and there's
one closing in 4hrs as I post and current price is 5750. Both are brand new unopened. Also saw an opened one close at 5100 Yen.