The Polish Virtual Library of Science (which contains Banach’s book) has several other interesting mathematical works in English, French, German, and Polish. They have the archives for several journals, including Studia Mathematica. They also have several older but famous monographs, such as Zygmund’s Trigonometric Series.

Stefan Banach’s famous monograph, Théorie des opérations linéaires, which gave birth to the field of Banach spaces, is available online.