I keep a literary diary. I fill in the following order:
Author, country, year, (century).
Notes about the author. Briefly about the author from Wikipedia (about how this work appeared) .
And then I fill it in from memory. If there are several chapters, then I write them down at intervals. If this is a play, then the names of the heroes fit in right away, and then I also make notes. I also try to write not only the plot, but also my own analysis. I write in the form of a synopsis (that is, in the present tense).
It develops memory very much and helps to plunge into my own experiences - in those that were exactly at the time of reading. Someone might say that a summary can be found on the Internet, but believe me, this is not at all the same.
"How to read books", by Sergey Innokentyevich Povarnin. A website was made according to his method - Buckella. Try it. There, the reading process is divided into stages: choosing a book, acquaintance, etc. and for each stage, you need to make notes by formulating your thoughts.
I would like to add to the previous answer the words of Ralph Waldo Emerson: "I remember the books I read no more than the food I ate, but they helped me to succeed (as a person), (helped to shape my personality)." p>
"I cannot remember the books I've read any more than the meals I have eaten; even so, they have made me".
So forgetting is natural)
On my own I can add that, according to my experience, re-reading the book helps to remember. The information somehow fits in the head. Something is still forgotten, of course, but a lot remains in memory.
First, you need to read thoughtfully, at a moderate pace, delving into every word. Secondly, it is best to read the book in printed form, and not in electronic version. this makes it possible to make notes with a pencil, mark interesting moments and place bookmarks somewhere (and from an aesthetic point of view, it is somehow more pleasant, more sincere, or something). Thirdly, reading should be interesting to you, then you will have the desire to live life with the heroes, empathize, sympathize, fantasize, reflect on certain topics touched upon in the work. This is with regard to fiction. If this is some kind of scientific literature, then you can somehow outline the key points, and then, based on your notes, remember everything.
The most important thing is that you love reading, have fun and start simple. Good luck!