James L. Throne 
Understanding Thermoforming [PDF ebook] 

Modern thermoforming practice is a balance of practical experience and the application of engineering principles. This very practical book introduces the process, its tools and machinery, and the commonly used materials to novices and practicing engineers alike.
Contents:
General Forming Concepts
Part Design
Machinery for the Thick-Gauge Forming Process
Machinery for the Light-Gauge Forming Process
Machines for Other Applications
Molds and Mold Design
Methods of Heating Sheet
Sheet Stretching and Cooling
Trimming
Polymers and Plastics
Quality Control
Comparison with Other Technologies
Pragmatic Aspects of Thermoforming