The book Build Your Own Earth Oven has a nice general approach. They frame earth oven building as a community-oriented project, and describe a variety of designs, using easily-sourced materials, tailored to the interests of the builders.
One suggestion in the book is to gain familiarity with the materials and the design by building a few small experimental oven projects.
Handprint Press article on cob construction
This book looks like the standard: Build your own cob oven, by Denzer et al