I found this YouTube video by Austin McConnell quite enlightening. I was unaware that there had ever been any letters other than the 26 standard ones that we know today. The funny thing is, I know I’ve seen some of these in old books and texts and wondered what the hell they were supposed to be, but never got around to looking them up. At least one of them, the Ampersand, I was aware of and that’s mainly because of its usage in programming and search filtering.
Interestingly enough, quite a few of these are a part of the standard character sets that can be typed on your computer. Some of them have been repurposed for other uses, but they’re still there.