Sonic the Hedgehog Modern Style Guide
From Sonic Retro
Sonic the Hedgehog Modern Style Guide is a book intended for use by merchandise producers used in the 2010s. It includes many new 2D and 3D art assets. Following Sonic art assets DVD, it also contains all of its artworks except for "classic" content which was instead used in its counterpart.
Scans and digital versions
Digital version