The MM* subdirectories contain single-tune files from the four volumes, with Makefiles to combine them into large files that have the tunes in the same order as the original books. In each of the subdirectories, the single-tune files have names of the form VN_Title.abc, where V is the volume number, N is the tune's number within the collection, and Title is the tune's title in the common mixed-case form. The tunes that are transcribed differently for ABC versions 1/2 also have "-V1" or "-V2" after the title, before the ".abc" suffix.
The transcriptions were done from August 2016 to March 2017, in my spare time. The sources were copies of the books published in the last couple of decades. I was unable to find images of the books online, which was a bit odd since so many of the old collections are now available as scanned images. If anyone has oiginal editions, and has the time, comparing them with the transcriptions here would be a real contribution to this effort.
Note that in book 3, there are no tunes 324-333 in the currently published editions. There's no missing page; the numbering just jumps from tune 323 at the bottom of page 35 to tune 334 at the top of page 36. It would help to know whether the original publication showed this anomaly.
The MM*.abc files here contain all the tunes in each of the four Merry Melodies books, in the original order. The "-V1" and "-V2" in their names indicate that some tunes are transcribed differently for the versions 1.* and 2.* of the ABC definitions. Version 2 supports things like two voices on a staff, crescendo/diminuendo symbols, and a few other features.
The transcriptions were proofread by formatting the *V1.abc files via my "jcabc2ps" clone of the original abc2ps program, and converting the *V2.abc files via Jef Moine's "abcm2ps" program, and comparing the resulting PDF files with the originals. This was done using a rather small scale, and many musicians will want them displayed or printed in a larger scale. This is best done by downloading the ABC files and reformatting them as needed. The originals have most of the tunes in a single staff, in a very small size, on pages with a "landscape" format (wider than tall). This isn't musically significant, of course, and you should reformat them however is best for your readers' eyes. Or convert them to MP3 for people who prefer to learn by ear.
Some tools to do things in directories of .abc tune files:|
Tune lister — Collection lister