Unpublished letters from Euler to d'Alembert
According to Eneström (as of 1913), the original letters may be found in the library of the Paris Academy of Sciences.
A partial English translation appeared in "History of the Exponential and Logarithmic Concepts. III. The Creation of a Theory of Logarithms of Complex Numbers by Euler," Amer. Math. Monthly 20, No. 3, pp. 75-84. Available online at Archive.org.
The six letters are dated: 15 April 1747 (p. 137-140), 19 August 1747 (p. 140-142), 30 December 1747 (p. 143), 28 September 1748 (p. 144-145), 27 December 1748 (p. 146-147), with one 1749 letter left undated (p. 147-148).
Original Source Citation
Bullettino di bibliografia e di storia delle scienze matematiche e fisiche, Volume 19, pp. 136-148.