As I mentioned, I have the master tapes, so I think it might be better & cheaper to transfer them onto a format that is easier to market. Ideally, from home, A long time ago, I sold the 3 audio-cassettes by post!.
So if you could explain in simple language the process + format + cost idea, that would really help me understand?
1) Do I need to take the master tapes to a recording studio?
When you say "master tape", are you actually talking about the cassettes, or are they on reel to reel tape, which is what most people call a "master" for quality purposes. While the actual transfer process is the same, A cassette deck is probably more readily available. As Rob said, as long as the tape is in good shape, transferring to a digital format is simply a matter of playing the tape and sending the output to a computer and recording the signal. Save the audio file as a .WAV file and that becomes the new "master". You can make multiple copies to store in a safe place, and unlike tape, they will all be equal in quality. Once saved, you can convert it to any other format is a few seconds. The free program Audacity can handle that function easily. For simple voice, an MP3 is perfect. You can also make CDs to sell. You can edit the original file into individual stories, or put them all on a disc like songs on an album.
There are studios that can perform the service, but will charge, often by the hour, for such conversions, but it's a job that most computer audio hobbyists could do easily.
If you want to add multiple images, then you are looking at a slideshow type of software. You can publish as a MP4 (video) format with the photos synced to the audio. Or that could be put on DVD to sell physical copies. This is a more involved process, and you'll need images. That may involve licensing if you don't have originals.
3) What does ‘carrying’ it mean?
I think he's just mentioning a USB stick as a medium to hold the audio tracks. Once in digital file format, they can be put on CD, USB memory stick, SD flash drives, or uploaded to places like Youtube, Apple I-Tunes, Spotify, etc.
Burning audio CDs just requires a computer with a CD burner and software. You could get fancy and have CDs with printable labels, which means you need the special printer but it could give a more "professional" look. CD burners are relativley cheap, the special inkjet printers will cost several hundred ($ or £). Inkjet printable CD-Rs should cost less than a £ a piece in lots of 50. Or you could have a commercial duplicator make CDs. It all depends on how much you want to invest and how big you think the market will be.
If you have a cassette deck and a computer with a mic/line input, you could go as simply as buying a cord to take the output of the cassette deck into the computer. Use Audacity to capture the sound (you will need to adjust the cassette's output to give a clean signal to the computer) and save the file. Once in the computer, there are ways to manipulate the audio for quality purposes, and I'm sure one of us here could help with that. Then you could decide if you want to pursue the project further.