Published by Birkhauser and the
Euler Commission of Switerland, the
Opera Omnia is the definitive printed source for Euler's works. Publication began in
1911, and to date 76 volumes have been published, comprising almost all of Euler's works.
Put simply, the Opera Omnia is the authoritative source of Euler's works. Not only do his writings appear in neatly typeset, edited form, but each volume also includes commentaries, some of them very lengthy and very scholarly, on those of Euler's works in the volume.
In addition to publishing Euler's published works, the Opera Omnia has published three volumes of Euler's correspondence (see link), and has several more volumes in preparation.
Any serious Euler scholarship must take into account this invaluable resource put together by the Euler Commission. The Opera Omnia can be found in many large research libraries, and is well worth the time spent with it.
Follow this link for information on purchasing volumes of the Opera Omnia.