In 1994, the late James E. Walsh reported that the Harvard collection of fifteenth-century printed books, the third largest in North America, "comprises 3,517 editions in 4,187 copies." Ten years later the count has risen to 3,627 editions in 4,389 copies. Walsh's pioneering catalogue was published in five volumes between 1991 and 1997. This supplement describes 202 new incunabula at Harvard: 67 complete or nearly complete copies and 135 single leaves or fragments, representing a total of 173 editions, including 110 not in Walsh's original five volumes.

The initial section of the First Supplement consists of selected additions and corrections to the Walsh catalogue. The following section, "New Entries," details single leaves and fragments which were previously given only highly selective coverage. The supplement concludes with cumulative references, indices, and concordances. The apparatus follows the Walsh model, and the book is designed to be used both on its own and in conjunction with the five original volumes.