Dewey:
No, I have never seen either the Einstein/Infeld volume or the
Sommerfeld series in Dover catalogs. On the subject of your quote,
I unashamedly invite you to peruse my article in the current issue
of Quantum (Nov/Dec, 1996).
Jack:
A few years back I called Academic Press to get one of the Sommerfeld
volumes. My memory says that only one of the volumes was still in print
and at a "rare book" price (my interpretation of their quote). I am
going to make a new inquiry.