From my bookshelf: books on illustration.
Since illustration encompasses a vast array of styles and techniques, books on the subject tend to be equally varied. However, unlike books on graphic design, books on illustration tend to get less dated—unless they refer to computer software or hardware.
Here are some of my recommended books on illustration and related subjects:
Pen and Ink Techniques
Rendering In Pen And Ink (ISBN:978-0-8230-4529-3)
Mixed Media Techniques
Painting With Watercolor, Pen & Ink (ISBN:978-1-58180-265-8)
Pencil Sketching Techniques
Drawing Realistic Textures In Pencil (ISBN:978-0-89134-868-9)
Coloured Pencil Techniques
Creating Textures In Colored Pencil (ISBN:978-1-4403-0850-5)
Cartoon 360° – Secrets to Drawing Cartoon People and Poses in 3-D