Collins, John F. A Primer of Ecclesiastical Latin. Washington D.C.: The Catholic University of America Press, 1991 .
The book is exactly what the title claims. It introduces the student to the structure of the Latin language with an eye towards medieval church sources. It’s set up similar to most Latin textbooks, except that the practice exercises have a more “churchly” flavor.
If the student has had some other foreign language (preferably Greek or German), then this book won’t present any difficulties as concepts like “declensions” will be quite familiar. The beginning of the book is quite user-friendly. The practice exercises are very basic and there aren’t many. By the end of the book, however, the student will be translating 50-70 one sentence verses from the Vulgate.
The book claims that you can translate medieval Latin after one year of study. I think that is mostly true. However, the book needs supplementing by other Latin texts. While there are useful morphology charts at the end of the book, by the last part of the book the author stops translating new concepts and verb forms. Other texts like Henle’s will fix this quite nicely.