Mark,
Maxwell only worked really in the Philly and New York areas. Here is a breakdown by city.
Philly
Gulph Mills - renovated 5 holes on the course and most of those changes are in place, but Gil Hanse, I believe recently made some changes and Tom Paul can elaborate on how much of Maxwell's work is still in place.
Melrose - the only complete course that Maxwell ever constructed in Philly. Pieces of the original design remain, but the Parkway construciton eliminated some great holes from the original routing.
Philly CC - Some of the greens still retain some of his recontouring and the tenth hole is still laid out in the same manner that he rerouted it.
Pine Valley - The 8th and 9th greens that he built are still in place and the bunkers on the right side of the 5th hole are still in place.
Saucon Valley (Old) - Maxwell only redid two holes there and they are still in place. I think for the club they play as the 11th and 12th now. I would have to check on that.
He did some other minor work that has been obliterated since at Flourtown and it is believed that he did some work at Merion, but not much else is known about that other than a lot of references to him doing something there.
Maxwell also maintained an office in Elkins Park, so I believe there was more work in the region than is currently unknown. Or else I doubt that a frugal man would have had an office.
New York - This area is much more fuzzy. Most of the work that Maxwell did was on Long Island, with a couple of exceptions. Most of my information on the Long Island came from Vernon Woods, Perry's nephew as he accompanied him to Long Island the summer Perry worked in New York.
Rockaway HC - Some work to repair the course from the 1938 Hurricane. Not sure how much is still there as the course has been touched since then.
Maidstone - Some believe he built the current 17th green. Strong evidence supports that he did some consulting work there in the wake of the hurricane and even some minor work after consulting.
National - He did some work there in the mid 30s prior to the hurricane, but no one knows for sure what. But based on the work, Macdonald hired him to work at the Links Club and referred him to several other clubs in New York later on after the hurricane ran through.
Huntington CC - Not sure the extent of the work
Perhaps there were others as well. Vernon remembered going to several places, but the only one that he could really remember the name of was Rockaway.
Maxwell also worked at Westchester CC. If you want to see the details of what he did in the late 30s there, read the In My Opinion piece on the course on this site. Good stuff there.
There was also a belief in the family that he did some work at some course in Connecticut, but no one seemed to know what it was and I haven't been able to track down any leads to such work.
Hope this helps.