-- Is there a source of information by plant type for proper spacing of plants using this method? --

I am not sure what you are asking. You are on the Sq Ft Gardening forum and asking if there is a spacing guideline for deep raised beds.

Of course there is. You can go with the spacing in the Square Foot Gardening Book or the spacing in the Cubed Foot Gardening book which is based upon Sq Ft Gardening but uses 12" rather than 6" beds. Alternately you could go with "WORD, The Vegetable Gardener's Bible" which focuses on deep dug (into the ground) raised beds.

In the end it is just different ways to the same end. Take your pick.

Thank you for the titles. I'll check them out :)

There is also a wonderful FAQ in this forum which answers many questions.

Or you could look here -
http://www.squarefootgardening.com/

or maybe even search the forum -
http://forums.gardenweb.com/forums/sqfoot/#search

Good Luck,
Gumby_CT

Here is a link that might be useful: FAQ-What are recommended plant spacings?

you can also learn more about "intensive gardening methods" which are informative for the square foot gardener. here is a good link:

http://cals.arizona.edu/pubs/garden/mg/vegetable/intensive.html