You can find The Secret of Eldorado, which is the documentary that I originally watched that first got me interested in biochar and terra preta at:
There isn't much technical information about terra preta besides the fact that it is typically rich in biochar and pottery shards, and seems to be present in multiple areas of the Amazonian basin. But, watching this documentary will give you enough of a background on the story to bore anyone that will listen with the relationship between conquistadors, charcoal, and that barrel you have been cutting and pounding on.
Living Web Farms has a bunch of different workshops on different subjects including a Biochar Workshop you can watch at: http://www.livingwebfarms.org/#/biochar-workshop/4581248499
There are a total of five different videos in the Biochar series, so you might want to watch all of them. There is a lot of different information, such as a slightly more involved version of a backyard retort to make biochar, a larger commercial version of a retort, how biochar works, etc.
There's a description of a SARE project dealing with some test plots and different TLUDs at: http://www.dyarrow.org/KAW/BiocharontheFarm.htm
One of the interesting details learned from these test plots is that applying as little as 500 lb. of biochar per acre can start to give desirable results.
Besides all that, I've been applying about half of a wheel barrow full of biochar at a time to an area that's about 100 square feet (I believe that comes out to be about 5 tons/acre). Over the last 8-10 years, I've covered the entire garden with bio char at a cumulative rate that's somewhere around 50-100 tons/acre.
When I first started, I made sure to "charge" that biochar with manure, compost, or even fertilizer. Now that my soil fertility has risen, I think I can get away with just applying the biochar to the garden (or I can start building another garden instead of applying more biochar where it really isn't needed anymore).
I haven't done any side-by-side comparisons to verify the the biochar is actually doing everything that I'm claiming it does, but the last soil test I had done on the garden showed I had a pH level of just over 7, the nitrogen, phosphorus, and potassium levels were at relatively high levels, calcium levels were high, and the water infiltration rates seem to have gone way up since I started applying biochar. I don't believe I would have those sort of soil test results from just applying manure, compost, or fertilizer.
Because of my experiences, I'm convinced that applying biochar works.