There is a recently published book written by Thomas Hudak which presents an overview of Tai dialects that were originally studied by William Gedney from data he collected beginning about fifty or sixty years ago. Gedney was a researcher who performed some of the first systematic studies of Tai languages using modern linguistic techniques.
The book gives a brief introduction to the geography and phonology of Tai languages, and then has short sections on each of the 19 dialects studied. You can get a good idea of the kind of information presented by looking at the preview of the book linked below. Following that, the rest of the book gives the data collected by Gedney, showing the phonological and tonal characteristics of each dialect. This part is actually the bulk of the book, containing lists of basic words with translations and phonetic representations.
Google books preview of Hudak's book
Various dialects studied are shown on this map.
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