Smyth's original 1920 work is now out of copyright and has been reprinted by a number of different companies. However, it was updated in 1956 by Gordan Messing and published by Harvard University Press. This version is still under copyright and only published by Harvard UP. This is the version that most people refer to when they sing the praises of Smyth, and the one most will encourage you to get.
You can get it here