The Tanka Teachers Guide can be downloaded for free at
In addition, Modern English Tanka journal can be read free online. In addition to the many poems in it, you will find scholarly articles, explaining and analyzing different elements about tanka
Tanka Online has poetry and articles and a recommended reading list
The Tanka Society of America has a web page with some articles and some tanka
TankaCentral.com aims to be the megasite for tanka information. It has a Reading Room with reference material and poetry, including tanka being read aloud by poets (anyone can contribute sound files).
The Anglo-Japanese Tanka Society publishes both tanka poetry and articles about tanka