The research tools available to scholars in the history of the book and the history of reading are in many cases based upon decades-old calendars and catalogs. Although the Pollard and Redgrave Short-title catalogue of printed books (STC) and its companion, the online English Short-title Catalogue (ESTC), are reasonably comprehensive, users are tempted to overlook the plus-sign at the end of many catalog entries, and forget that it designates the existence of additional surviving copies. Even though digital technologies allow the researcher to locate multiple copies of a single work with greater ease than ever, many libraries have been slow to place their catalogs online, and some for various reasons have not begun the task. Even if the existence of a book is known, scholars may not actually consult it. Because each copy of an early printed book is distinct in terms of its provenance and the material traces of reading which it might preserve, however, these copies must be examined before scholars are to achieve anything close to a general history of the relationship between the printed book and human thought. Such a history is clearly desired, but in the early modern period, provenance and marginalia research are still in their infancy. Despite the value of several prominent studies in the history of reading, marginalia remain the great untapped category of evidence in book history research. Even scholars who seek copy-specific details in surviving fifteenth- and sixteenth-century printed books typically examine a small number of copies located at well-trodden archives. The British Library, Folger Shakespeare Library, and other major repositories certainly cannot be overlooked by the scholar seeking evidence of an early printed book’s reading and use. But they represent the tip of a vast iceberg of surviving copies. Scholars will never learn what these copies can tell unless they travel to the archives where they reside. This paper will bring together a series of case-studies based upon my own experience in obscure libraries. These fascinating finds include numerous copies of John Foxe’s Acts and Monuments annotated by sixteenth-century readers, a copy of Foxe turned into a Reformation scrapbook by a seventeenth-century poet, a papal presentation copy of Nicholas Sander’s De origine ac progressu schismatic anglicani that was probably never sent, and other books which offer valuable insights into how contemporary readers consumed and used early modern print. The essay’s intention is broader: to reveal the benefits to be gleaned from the use of bibliothecae obscurae as a research method. Libraries under consideration may include the now-dissolved St. Benedict’s Abbey library, Fort Augustus, Inverness-shire; the sixteenth-century Ipswich Town library, which survives intact in the headmaster’s office at Ipswich School; the library and archive of Stonyhurst College, Lancashire, which dates from the sixteenth century; and more.