The Hunchback of Westminster by William le Queux

Directly I saw the half-faded parchments I recognised that the cipher which those early Jesuit writers had used was a very simple one. This, I may explain to the uninitiated, is not so difficult a matter to observe as they might perhaps think. Some very simple considerations guided me to this conclusion. For instance, unlike the cipher in use by the founder of St Bruno, there were practically no figures in the manuscripts, and by that I gathered the cipher was complete in the alphabet in use, and did not refer to any book, like the institutes of St Bruno. The one thing that does puzzle and sometimes confound experts is to adopt a book, as Bruno Delganni had done, and then to put in the record simply figures that related to letters in this volume.

That, however, is not always quite safe. The expert looks in the manuscript for the letter that appears most often, and he decides, perhaps, that that is "e," the second "i," and so on according to the letters that are most frequently used in the language in which he believes the document to be written, and by this simple means he can often construct the entire code alphabet.