Mail-Art Books

I found that if you google 'mail art' often, then the nail art auto correct stops appearing. As to mail art, there aren't any precise guidelines on how to make it. Technically, it can involve virtually any medium and applies to anything that can be mailed. Practical limitations, though, may apply. People have tested the postal system by attempting to mail all kinds of things. Maybe these links will help you. They offer brief overviews of what has been done: How Did Mail Art Develop into a Global Art Movement and Mail Art and Fluxus: An Antic Exhibition from 1982.

Fact is that there is no single way of creating mail art. John mentioned already two good references, and also the wikipedia page on mail art provides a good overview.

There are some books about creating postal art (like Jennie Hinchcliff's book 'Good mail day'), I think that is what you are looking for?

To my opinion, one can distinguish several forms of mail art, of which I think there are two main kinds to differentiate: autonomous art (rooted in fluxus but also in Van Gogh's illustrated letters) on one hand, and craft-art on the other hand.

The 'autonomous' art can be made in all kinds of ways (drawing/painting, collages, weird objects sent through the mail, etcetera). It can be beautiful, it can be ugly, or something inbetween. For the artist, creating art is a way of life, a part of their identity, and he/she creates art in his/her own way. Some have an art school background, many haven't, but they need no tutorial because the urge to create and send art comes from deep inside.

Mail art made in a craftmanship way also can be created in all kinds of ways. There might be some links to the way scrapbook and bujo pages are made, and there are several tutorials on instagram and possibly also on youtube (al least many diy's and tutorials on scrapbooks, which can be useful for mail art, too). The people I know who work this way consider creating mail art as a nice hobby, not necessarily as part of their identity.

Of course you are free to start your own channel, and I think it can be helpful for the secondly mentioned. On the other hand, the autonomous mail artists will find their inspiration anywhere. 

There are many interesting parts within mail art. The ATC's, the cinderellas/artistamps, the rubberstamps (there are a lot of books and tutorials on rubber stamp carving), the asemic writing, visual poetry and many things more.

Testing the postal system, as John mentioned, has been tried already 120 years ago by W. Reginald Bray (see the IUOMA group and the website), and also nowadays there are several interesting attempts, like the great and fun project by Harriet Russell. Her book 'Envelopes' I think inspiring, too (here and here some examples).
And of course the Post companies add something to the mail, which can be annoying, or which we can consider as part of the art.

Ran into this book in a thrift store today. Very interesting and well designed. 

https://www.youtube.com/watch?app=desktop&v=Gpc1LXl90Rs