Bob Hyman has produced many excellent clinics and articles. Some are on this web site:
Background Trees (PDF)
Choctaw, Oklahoma, & Gulf Railroad (PDF, 5MB)
Many more clinics can be found on Bob's own web site:
Modular Framing for Dioramas and Layouts
Conifers from Yucca Stalks
Trestles
Mind Games
Anatomy of a Module