When encountering a seemingly difficult book, I ask myself two questions - is it the right book in the sequence of what I am striving to learn and do I have enough background knowledge to understand it.

When watching lectures on YouTube on software engineering, I note which books those presenters recommend. I also read works they previously wrote because the presentation tends to be an overview of their books.

The book may be essential reading, but it likely assumes prerequisite knowledge if it seems difficult.

For example, Strunk and White is an essential book for writing better in English. I had to read additional books to understand the assumed concepts.