Here's a useful site I don't recall having seen before. It's a bibliography set up as a portal to open access versions of a broad range of numismatic books. It seems to be the work of an individual (T. Verspag?) who updates it periodically but not regularly.

The purpose of the project Digital Library Numis (DLN) is to establish a major portal and digital repository for open access, e-books and e-papers, regarding coins and medals, the history of money, and otherwise related subjects.

The core body entails a broad variety of many rare and often beautifully illustrated numismatic classics produced from the 16th until the early 20th century. but we also offer an extensive survey of contemporary numismatic publications.

Digital Library Numis is particularly useful as a additional research tool for professional numismatists, other academics interested in the field, branch libraries and coin collectors.