In light of current events --- Happy (Belated) Birthday Nino!!!!! --- I have a few selections of the Tekkon Kinkreet artbooks. The artbooks are amazing (though expensive!), and there is a "White Side" and "Black Side". Shiro's part contains images of the colored CG work, including landscapes organized into the districts of Treasure Town. Black's part contains the hand-drawn illustrations as well as pencil sketches of the environments. Although nothing is specifically Nino, the whole feel of the art captures the playfulness, innocence, pain, and complexity of Shiro and Kuro.
(I remember an interview that once said that Nino seriously felt Kuro's pain in the latter scenes, and that when he was doing the seiyuu, he ended up gripping his arm so violently that he gave himself bruises.)
For appreciators of the art, definitely get the books; definitely worth it! The covers of the books themselves are amazing because they don't just have the bookcover, but have art imprinted across the covers (see last two images).
Treasure Town Map (Shiro)
City Rendering (Shiro)
Shiro and Kuro (Shiro)
The Park (Shiro)
Kuro Flying (Shiro)
Shiro's Drawings (Shiro)
Shiro's Drawings... lol@Fujimura (Shiro)
Corrupted Town (Shiro)
Kuro at Beach Rakuen (Shiro)
Types of Fish (Shiro)
City Rendering (Kuro)
Shiro's Stabbing (Kuro)
Kuro Breaks (Kuro)
Shiro Saves Kuro (Kuro)
Under the Bridge Sketch (Kuro)
Bathhouse Sketch (Kuro)
Temple Sketch (Kuro)
Hardcover Illustration (Shiro)
Hardcover Illustration (Kuro) <--- love this one!