You probably noticed some conflicting information and that's going to happen a lot when it comes to reading and studying. People develop habits that work for them and so will you. There's no one right way to read history but there are some strategies you can use to help your brain make connections. (Basically, that's what's happening when we "retain" information; the new information hooks onto information we already know.)
First, consider taking a blank sheet of paper and writing down everything you think you know about The Civil War. It can be a list of words, phrases, dates, and names or a concept map but the idea is to dump everything you think you know about a topic before you read. Then, as you read, consider adding information that surprises you or catches your attention. Basically, if you feel yourself going, "huh, I never knew that!" consider writing that thing down. Your goal is to start to make connections. As you read, you may notice that the things you wrote down fit into categories and you may find those categories change the more you read. So, rather than taking notes as if you're preparing for a test, think of it like taking notes to help your brain keep things straight.
Second, set a purpose each time you sit to read. The author set a purpose when they wrote their book and as a reader, you can choose how you engage with that. Are you reading to understand the argument they're making? Or are you reading because their argument is different than what you've read in other places? Be sure to read the introduction to every book you read - that's typically where the author articulates their writing goals. Get comfortable checking sources. Most authors will tell you how they know the thing they claim to know in either a citation, footnote, or endnote. Take some time to follow up on the author's claim. Even those the won the Pulitzer. This means going back to primary sources and spending some time with the original documents. Your goal isn't the check the author's work, but rather, to look through their eyes and see if you see the same thing as them when you look at the primary source (if it's available to you.) Some people like to read footnotes as soon as they appear, others read through a chapter or article and then read footnotes on a second read. That's a matter of personal preference.
With regards to dates and names, some dates and names are going to come up over and over again and eventually, you'll retain them because you've got so much background information around them, they will stick. However, if one of your goals is to learn more about the people of the Civil War, you may find it helpful to jot their names in a notebook. I have a cheap alphabetical notebook where I write down new-to-me names along with the name of the book or article where I learned about them. Creating a timeline is also an effective strategy - especially for events with a discrete beginning and end. You can think about creating timelines for individuals and their movements or for events and when different individuals appeared. Any approach is fine and again, you'll find what feels most comfortable for you.
Highlighting is a dramatically over-rated activity - but it can be helpful to underline things that fit with your understanding and circle things that don't fit with your understanding and then consider reading different texts to get a better understanding. I often write key words in the top margin of books as I read them - just one or two words that summarize that page or to remind myself of something new I learned from that page that I didn't know before reading. These notes act like markers for me when I'm doing a bunch of reading and can't remember if I read a page or skimmed it looking for something in particular.
Finally, enjoy it. This goes back to purpose - find what sparks for you and follow it. In other words, if you find yourself interested in how contraband camps worked during the War, it's fine to step away from McPherson's book and read books and articles about the camps. If you're bored by dense sections detailing tactics, revisit your purpose and be clear as to why you're forcing yourself to read it. Know that historians are often very specialized. I know a historian who just published a book on the history of union soldiers who were disabled as a result of injuries sustained during the war. So, she's a Civil War historian, but looks at a very specific aspect of the war.
Ok. One more. Historians are people - and people carry our biases with us and no one historian can tell the entire story. Whenever you can, try to consider whose story isn't being told or who is being left off the page in the book you're reading and then consider looking for books on those people. Going back to the historian I mentioned, she wanted to write her book because too often historians talked about "casualties" but didn't often tell the story of what happened to the soldiers after the war. Consider keeping a page in a notebook for questions such as, "I wonder what happened to men who deserted?" or "How did the enslaved people feel when Union troops arrived?" These questions can lead to your next book.
Reading is active. The more you can do to help you brain make connections between what you're reading and stuff you already know, the more you'll remember.
One of the big tasks in retaining your knowledge in history is making sure you understand what is happening and going on. Making a timeline and writing down specific dates is one way to help you not get confused on when things happen. One big mistake that many people make, is highlighting random words that seem big or just any definition. Annotations are extremely helpful in understanding what is going on within your text. There are also plenty of YouTube videos that you can watch to help you get a better understanding onto what is going on. After you understand the knowledge in the text, go back to what you learned and reread it and teach it to someone else or pretend you are. Teaching something to someone else will help you get a better understanding of the material and it will stick in your head. Another helpful tip is to stay on top of things. Procrastination is something that everyone struggles with, so create better study habits for yourself and start now. Take it page by page and annotate everything you read. You got it!
Master's History Student here I have a few tips that might help you.
I make it a story. It all fits together into one big story of humanity so it all IS part of one big story. The more you learn, the more what you learn fits together, the easier it is to remember what you learned.
Remember that the people you're learning about aren't chess pieces getting poked around the board, they are humans who had the same needs and motivations that you do - creature comforts, food, sex, money, fame, power. They did what they did for a reason. When you find the reasons, you find the plot. T
hen figure out cause and effect, which is also plot but in a bigger way. School teaches you "big man" history, the idea that nothing happens unless an important rich white guy says so, but that's not how it happens. Political movements, thought, and technology are influenced by a milieu of waxing and waning culture, thought, and other factors. Find the little guys, the women and brown people, the proletariat, and you find the reasons for change.
[Very short answer, sorry...]
I'd say structure, structure, structure.
First, memorize the table of contents by heart and figure out the structure of the arguments. Then, memorize key words and their definition. The more you do it, the more you can memorize information and organize it properly into your brain. It will be like drawers you can open and close whenever you want. In the end, you don't remember always the information clearly, but you end up remembering where you saw it and how to access it again.
Go for the structure of the book, or syllabus, or whatever, then you'll remember the specifics much more easily.
Good luck! :-)