Northern Thai is also known as 'muang', 'yuan' and, I think, 'yanok'. The northern Thai alphabet is based on the Mon script whereas Thai is based on Khmer, which is why the northern language has so many circled letters. Hans Penth, a linguist at Chiang Mai University has written a book about the northern Thai language that looks at old evidence from stone inscriptions. The Chiang Mai Teacher's College is heavily involved in preserving the northern language and script. Look for Aroonrut Wiechienkeeo.
I was able to study northern Thai for months at Chiang Mai University back in the mid-1980s under the University of Wisconsin's, 'College Year in Thailand Programme'.
Under the dynamic leadership of King Tilokarat (15th century), Chiang Mai became the cultural centre of a world that included the area down to Sukhothai, Nan, Phayao, Luang Phrabang in Laos, the Sip-song-panna region of China (the Chinese call these Tai speakers, Dai), the Shan States of Burma, and Ahom in Assam, India. Tilokarat also hosted a world Buddhist Conference in Chiang Mai and had Wat Chet Yot(d) built in Chiang Mai to host it. His ashes are buried at the wat. He was able to beat back many an attack from his contemporary, King Trailok, in Ayutthaya. Trailok also successful kept Tilokarat's northern forces at bay when they attacked Ayutthaya. There is a poem from sometime in the 15th or 16th century about the Chiang Mai - Ayutthaya wars.