I have written about the National Radio Institute before. They were a correspondence school focusing on radio. As with most of this blogs content, it no longer exists. I'll quote myself rather than rehash it all anew.
" In 1968 Mcgraw -Hill purchased it and changed the name to NRI Schools. They ceased operations relatively recently. A Smith bio states they closed up in 1999, a learn-at-home directory gives it as March of 2002."NRI struggled under Mcgraw Hill. In the 1970s they had the foresight to offer computer courses, but Mcgraw never successfully sold their text books to other schools, their primary business. Their phase-out in the 1990s was inevitable.
This first one "HOW TO START A SPARE TIME RADIO BUSINESS" calls itself a reference book, both terms being fairly dubious at 29 pages long. It's number "22X" in a series of unknown size or duration. It covers sale of radios, advertising and emphasizes repair service contracts. It has been out of print surely since the 1950s and is so obsolete it will never be in print again. No one repairs radios now, many stereos now emphasize their Aux inputs or better yet, USB.
You can download it