The American Mathematical Society offers some of their titles online for free.

AMS History of Mathematics, Volume 1: A Century of Mathematics in America, Part I, Edited by Peter Duren, with the assistance of Richard A. Askey and Uta C. Merzbach
The first 100 years of the AMS: profiles of mathematicians, the mathematical community during the world wars, the growth of mathematical fields...

AMS History of Mathematics, Volume 2: A Century of Mathematics in America, Part II, edited by Peter Duren, with the assistance of Richard A. Askey and Uta C. Merzbach
The histories of mathematical activity at the major U.S. institutions; the influence of women in mathematics; the flowering of applied mathematics...

AMS History of Mathematics, Volume 3: A Century of Mathematics in America, Part III , edited by Peter Duren, with the assistance of Richard A. Askey and Uta C. Merzbach
The mathematics and mathematical personalities in some of the nation's prominent departments.

A Semicentennial History of the American Mathematical Society, 1888-1938 Volume 1, by Raymond Clare Archibald
The first fifty years of the AMS: the founding, Bulletin of the AMS, meetings, officers, etc.

Semicentennial Addresses of the American Mathematical Society, Volume II
The second semi-cenntennial volume: developments and trends in fields including differential equations, harmonic analysis, geometry, topology and more.

Mathematics into the Twenty-First Century, Felix E. Browder, Editor
The proceedings of the AMS centennial symposium.

History of the Second Fifty Years, American Mathematical Society, 1939-1988, by Everett Pitcher
The second fifty years of the AMS: Mathematical Reviews®, primary journals, membership and dues, international meetings, policy committees, finances, etc.

Cornell University Library offers Historical Math Monographs as a free digital collection, there are over 500  titles which are made available through the Hathi Trust and Project Euclid. Use this link to browse the Cornell University Historical Math Monograph Collection.

Project Euclid, from Cornell University Library and Duke University Press, is an online platform that offers about 80% of it's content for free. It offers books and journals, both historical and current. Here is a link to Project Euclid's Open Access content.