Hi,
I read this book a while back, and it provides quite a bit of info on using earth tubes.
PASSIVE ANNUAL HEAT STORAGE, IMPROVING THE DESIGN OF EARTH SHELTERS
By Hait. Spiral bound 1983 original, 152 pages, 8.5 x 11 inches. Still the world's most advanced text on earth sheltering. PAHS heating & cooling systems, underground water control, convection powered earth tube ventilating systems, heat flow control, radiant, convective, conductive, "transportive." All PAHS publications below are supplementary to this book.
You can buy it from Hait's web site, but I got a copy from my interlibrary loan.
His website is: http://www.earthshelters.com/Catalog.html
His whole concept for storing heat in the dirt around the house under an insulation "umbrella" is pretty interesting, and unlike a lot of earth sheltered design concepts he has quite a bit of data to back it up (something like 50 temperature and humidity sensors surrounding his first test home). But, even if only the earth tubes part is of interest to you, there is quite a bit of both practical and design information on earth tubes in the book.
Gary