The "Revised Medieval Latin Word List" by R.E. Latham is an excellent reference work for students of British and Irish medieval history. Latin was the lingua franca of the medieval period, used in government, by the church, by scholars and chroniclers alike, with the result that the majority of our sources are written in the language. Many of these sources have now been published in English translation, but still at times students may want to refer to the original text, in which case this book will come in useful.
Included within the scope of this book are military, political, social, legal, administrative, theological, musical, heraldric and scientific terms - in particular those for which the meaning in the medieval period was substantially different from that in classical times. The definitions supplied are all derived from British and Irish sources dating between the 6th and the 17th centuries. This means that for each word there are often given a choice of translations according to date, and it is usually possible to find one that closely matches the period of whatever text is being studied.
I found it highly useful as part of my university dissertation in translating medieval chronicles, where certain words and terms are used that are not covered by a standard (classical) Latin dictionary. It should be noted that this book, though a useful companion, is not in itself a comprehensive Latin reference and that at times it will be necessary to resort to a standard dictionary - for example, the Chambers-Murray "Latin-English Dictionary" by Smith and Lockwood.
The "Revised Medieval Latin Word List" is a handy resource but still very much a specialist book, useful for students and for academics. For more casual readers of medieval history, it may be worth considering Christopher Corèdon's "A Dictionary of Medieval Terms and Phrases", a general reference which also includes Old and Middle English terms. Meanwhile, for someone interested in learning more about the language of medieval Latin itself, better choices might be Keith Sidwell's "Reading Medieval Latin" or E.A. Gooder's "Latin for Local History".