I am very interested in certain subjects which are quite complex (and not very mainstream), but I find it hard to learn about them since there seems not be a lot of introductory content about such subjects.
For example, I wanted to learn about computational intelligence and I picked up a book from the library, but quickly gave up reading it since it was way more complicated than I could handle (it was meant for graduate students). When reading the book, I couldn't tell exactly what I needed to learn for the book to make sense.
Also, I don't want books which are too mainstream (i.e. reading books like: "How AI will destroy the world")
So my question is: How can I learn about things which may appear to be unapproachable due to their difficulty/lack of documentation for beginners?