Remember, this isn't the be-all and end-all that it used to be. As Jonathan Kramnick reminds us in "The Way We Hire Now" (https://www.chronicle.com/article/the-way-we-hire-now/244467):
To get a grip on where things now stand, start with the fact that the MLA jobs list has lost its monopoly. The low cost and simplicity of doing things online has meant that advertisements now appear on any number of platforms, including The Chronicle, Interfolio, Higher Education Recruitment Consortium (HERC), HigherEdJobs, and well beyond.Don't forget the jobs at Insidehighered.com, too.
If you're looking for information on historical trends (and the now-infamous rosy vision of the Bowen report), here are some links.
MLA Report on the JIL 2016-17 (chart is from this source): https://www.mla.org/content/download/78816/2172744/Report-MLA-JIL-2016-17.pdf
Some older posts from this blog about job letters, still maybe useful:
The art of the job letter: http://notofgeneralinterest.blogspot.com/2007/10/art-of-job-letter.html
The art of the job letter redux, part I:
The art of the job letter redux, part II:
Good luck, everyone!