Sound of Wings Studio offers a series of workbooks on learning both gouache and scratchboard techniques. Many of the images on this site were done with these two mediums.
The scratchboard workbook covers cats, dogs, birds and flowers while the gouache workbooks cover botanical illustration, birds, animals, butterflies and moths and toads and turtles.
All the workbooks are geared toward depicting every little detail of its subjects, either with tiny little brushes to paint with gouache or a very sharp blade for scratchboard.
The demonstrations make it simple to learn the various techniques like blending and layering.
So find a quiet place and get ready to explore all the fascination of creating realistic illustrations.