I've followed the common approach of other ABC transcriptions of historic collections: The music has for the most part been transcribed as-is, "warts and all". Some obvious typos have been corrected (and others may have been introduced). Notation that can't be easily represented in ABC has mostly been ignored, though ABC has become good enough that not much need be omitted.
The two major omissions are: The fingerings aren't transcribed, and the staff breaks aren't preserved. I've followed the usual custom of putting most of the music in 2- or 4- or 8-bar lines, to limit line lengths to about 70 characters. This is to prevent to common sorts of damage done by email software that introduces newlines into longer lines. Backslashes have been used to combine these short lines into 4- or 8-bar staff lines, to get reasonably normal music on D4 and US letter pages.
An advantage of ABC is that it is very easy to reformat the music for different paper and screen sizes. This is especially useful for people with visual problems. But note that the files here are arranged for a fairly small print size. This is useful for archive and reference copies, but may not be good for some readers. You should reformat the staffs for a size and layout that works best for you. Such things aren't musical information, and don't matter in ABC transcriptions, but can be important when you need printouts that are easy to read.
The primary files here each contain a single tune. The file names are of the form VPPPN_TITLE.abc, where V is the volume number (1-3), PPP is the page number (10-288), and N is the tune's number within the page (1-4). N may also be 0, for the section headers and the headers of tune sets. This produces the same tune order as in the printed books.