The drawings for Hey, Rabbit! where done in pen & ink and watercolors.
I use an old orange wooden pen which is quite thick, so it fits well in my hand.
The metal nib is very fine, but flexible enough to get different thicknesses, even within the same line.
(This is a detail of a drawing I'm working on now for a different book.)
For Hey, Rabbit! I used black and sepia ink. Sepia is a nice grayish tone of brown. I don't know if they still produce it the same way, but originally sepia ink came from the ink sacs of cuttlefish.
I paint with four or five red (or Kolinsky) sable hair brushes of different sizes, with a round, pointy tip.
My watercolors are mostly Schmincke. They're the best I've tried so far.
Since I started painting with watercolors, I've always used Arches paper, "grain torchon", or rough.
And this is my desk, in my studio in Clinton Hill, Brooklyn: