Great stuff! I stumbled across this study of Modern South Arabian on a pdf a while ago: http://llacan.vjf.cnrs.fr/fichiers/S...MLanguages.pdf, although I seem to remember someone saying ESA is actually closer to Ge'ez (classical Ethiopian) than these languages. It's all rather technical, but probably easier to understand if you speak a Semitic language such as Arabic already (which I obviously don't)