To my surprise, I have found that it is sometimes hard to find classic papers of the physics titans of last century that were originally published in German (i.e. from the time before the Nazis killed off Germany's extraordinary place in science along with most of its Jewish citizens).
For instance, although Einstein found refuge in the United States, and his English was quite good, all of his most important work was published in German. When a new handwritten manuscript of his last great contribution to physics surfaced I could not find an English version. Although it was written as a continuation of one of Bose's papers that was originally authored in English. Einstein endorsed Bose's work and translated his paper into German to give this great physicist his dues. He then subsequently built on Bose's important theoretical ideas in two follow up papers.
These three publications will be the first of the lost papers to be featured here:
(Plancks Gesetz und Lichtquantenhypothese)
Zeitschrift für Physik (1924)
Equations numbered, Corrected formatting errors (formula 14 incorrect in – Ref.  contains correct version)
Quantum Theory of Ideal Monoatomic Gases - Part 1 (external link to translation) and Part 2
(Quantentheorie des einatomigen idealen Gases - erste und zweite Abhandlung)
Sitzungsberichte der Preussischen Akademie der Wissenschaften (1924/1925)
New translation, handwritten manuscript superseded print version when there were discrepancies.
The beauty of these papers lies in the fact that they are very accessible. Unlike Einstein's work on General Relativity, the math required is within reach of an aspiring High School student with a penchant for advanced calculus.
Nevertheless, the work is of the uttermost significance. As Bose himself put it when writing to Einstein:
Respected Sir, I have ventured to send you the accompanying article for your perusal and opinion. You will see that I have tried to deduce the coefficient .. in Planck's law independent of classical electrodynamics.
Not only did Bose try, but he succeeded spectacularly, deriving the law from which quantum mechanics sprang without any recourse to classical physics.
When Einstein then extended this idea to describe the monoatomic ideal gas, he deduced the existence of an exotic state of matter that is now know as Bose-Einstein condensate. Neither Bose nor Einstein would live to see this confirmed experimentally. It took until 1995 for technology to catch up to the theoretical insight of these titans of physics.
[Translations of Einstein's follow up papers will be linked in the table above as soon as the work is completed - over the course of the next couple of weeks.]