I'm always puzzled by posts claiming they are unable to read or understand books. You have the knowledge within you to understand anything. Nonfiction especially has to be completely transparent and faithful to the subject. A great recent nonfiction book is The White Devil's Daughters, which covers human trafficking in San Francisco. It's clear, carefully written, features a coherent history and the chapters are punctual. It is telling you the purpose of the book with every word. You could also try a Naomi Klein book - she is very clear, persuasive and in love with everything she writes. And every nonfiction book is describing some shocking or unaccountable discrepancy between what we believe and the truth of the matter. Every book does that, really, but most pointedly nonfiction. I would just get lost in the facts of the book itself. The actual "purpose" and "importance" is something the reader has to decide.

And I don't think there would be much use in taking notes. Most information in a book just serves to frame the narrative overall. Most of the facts in a nonfiction book do not need to be committed to memory. A book should be able to convey a general theme or thesis that you CAN remember and remark on throughout the text.