Of real benefit not just to numismatists but to medievalists who deal with material culture, economy, politics and more, are the three catalogues of Byzantine and medieval coins (including those of Ostrogoths, Vandals and Lombards, plus later coins from Thessalonica, Trebizond and Nicaea) by Warwick W. Wroth (see my delicious links on the right hand side). They are all now well out of copyright and you can at least read them via the Internet Archive which leads you to Google Books. They are allegedly available for download but I cannot seem to achieve this. I wonder if there is some residual rights problem as facsimiles of these volumes have also recently been published? However, you can at least consult them here. The quality of the reproduction is at least as good as the originals and so perfectly suitable for research purposes:
Where did they come from? The medieval ‘southern Italian’ collections of the British Museum
On 15 February 2005, I delivered a short paper to the Associazione Internazionale di Archeologia Classica (AIAC) at the Swedish Institute of Rome. The paper was largely based on my research at the British Museum where I was examining artefacts with a southern Italian provenance. My aim is to test and demonstrate methods with which museum objects can be interrogated as historical evidence.