I spent a bit of time this evening with the 1898 edition of John the Lydian, De mensibus, which I got from Google books. There is no table of contents, so I opened it in Adobe Acrobat pro and went through adding bookmarks.
The result was as follows. It’s far more useful as a guide to the work, and made me realise how book 4 is actually the largest portion of the work, larger than books 1-3 together. Getting a translation of book 1 (18 pages) will cost less than I paid for the March section of book 4 (23 pages).
It’s worth considering that it was a worthwhile exercise, just to edit this PDF in this fashion. I shall also OCR the praefatio, so that I can have the assistance of automated translation tools in trying to come to grips with the transmission of the text.