Tea - Legend, Life and Livelihood of India, is a coffee-table book by Gautam Prasad Baroowah. It's being distributed in this neck of the world by Cool Grove Press. It's not listed with the big online book sellers yet, but you can find more information at Cool Grove's Web site and in this piece that appeared in the Calcutta Telegraph a while back.
In Villages and Tea Gardens: Forty-three Years of Missionary Work in Assam, by O. L Swanson, is a somewhat more obscure work on the region. At last count there were two copies available at Amazon. You might have some luck finding it elsewhere, as well.
As we veer a little farther from the subject of tea, but still sticking with Assam, there's Ralph Izzard's The Hunt for the Buru. This one tells the tale of a 1948 expedition to Assam in search of a strange reptilian creature.
While we're veering, why not throw out a quick mention of Darjeeling: A Novel, by Bharti Kirchner? I know, it has nothing to do with Assam and perhaps all that much to do with tea, but there is at least a marginal connection to another great Indian tea-growing region.
Back to Assam. Here's an article from Tehalka - which appears to be a regional publication - on wildlife preservation and Kaziranga National Park.
Finally, from Assam.org, an article on the Lalung people of northeastern India.