Talking about frontier mathematics? Even the common for-mere-mortals version of mathematics is totally inaccessible to most people. Even to those who want to really learn it.
Take a good look at the textbooks that teach mathematics and tell me I am wrong.
Try university courses rather than books.
If there is any barrier, it is not intentional, I assure you. Math is just hard.
There are many amazing Youtube channels that explain complex math well, for example https://www.youtube.com/c/3blue1brown
... and quaternions?
The books exist and are possible to understand. They teach maths at most universities. You should try it, learning is fun.
please have a look at my profile before making silly comments. :)