There is also a book

The American Historical Company, Inc., founded 1877 : more than 75 years of historical publishing.
Author: American Historical Company.
Publisher: New York : American Historical Co., [1952]

that might give you clues as to what became of this firm -- i.e., who were the owners, did another publishing firm take over, etc.

See worldcat.org to see which libraries have it. You may be able to get it via InterLibrary Loan.