Essays on Early Medieval Mathematics is part of the Variorum Collected Studies series, which reproduces papers directly from their original (sometimes quite obscure) sources, preserving even the original pagination. (This is intended to make it easier to trace references to the papers, but it does mean that we get a book that isn't as pleasant to the eye.)
These are research papers, not expository writing, but this book is worth having in your library, even if it is strictly an undergraduate library, because all too many histories short-change the period in question, suggesting that nothing interesting happened in the early medieval period in Europe. The very first essay in the book, called "The Importance of the Latin Middle Ages for the Development of Mathematics", deals with this issue directly. It would make a great reading assignment for a history of mathematics course.
The rest of the book is more technical; among other things, it includes editions and translations (not always into English) of important historical sources. Given the price, few people will want to buy themselves a copy, but libraries that aim to have a good collection in the history of mathematics should have this one.
Fernando Q. Gouvêa is Carter Professor of Mathematics at Colby College in Waterville, ME.