Pomeranian language, the language of West Pomerania is extinct since 1950. It's only 60 years and I hope someday maybe pomeranian will have its own revival. The last dialect of pomeranian (except kashubian) was slovincian. Slovincian grammar is preserved in "Slovinzische Grammatik", compiled in 1903 by Friedrich Lorentz who in 1908-1912 also published "Slovinzisches Wörterbuch", a Slovincian dictionary. There are also "Slovinzische Texte" and those three books are completely enough to create full dictionary. They are also enough to learn this language, its grammar and pronounciation too! In my opinion it is one of the most preserved extinct language ever, especially because of full information about pronounciation and phonetical causes.