3rd party Terragen Resources
Geek at Play
An in-depth series of Terragen tutorials covering basic to advanced techniques
Advanced Tutorials by Graham Cristie
VFX artist Graham Cristie (Mission Impossible, Avengers: Infinity War, etc.) provides a Maya-to-Terragen script (Terraman) as well as advanced tutorials on Render Elements and Vector Displacement workflows.
New World Digital Art
New World Digital Art, founded by Frank Basinski. Now owned and managed by fellow VFX artist Danny Gordon. The NWDA Group is a 3d artist collective specializing in 3d environmental design and also offering professional freelance services from top industry VFX artists.
Have a look in their excellent Resources page for for great Terragen related tutorials & resources.