I recently finished my master's degree in education. During the process, I got to meet a lot of interesting people and few of them are some of my best friends at this point. I was recently talking to one of them about learning new material and how I remembered everything.
Specifically, she was curious if I remembered everything that I read and whether the memories formed on the first read or if I needed to read things multiple times as she did. My answer was yes and no. It depends. Declarative knowledge, or the knowledge we generally get from textbooks, as opposed to procedural knowledge requires a certain process for me to become comfortable with.
How Our Brains Work
Basically, I use a simple metaphor to understand how my brain works in terms of learning new information. I imagine a really overgrown jungle, bush, or forest area. Then, I imagine that I need to make a path through the overgrown vegetation. The first time that I hack my way through with a machete is going to be ugly. I'm going to barely make a dent in the plants and I'm probably going to be tired from all the hacking and maybe even a little beat up and bleeding from sharp and pointy thorns.
Needless to say, if I continue walking down the same path each day and hacking away at the same outgrowth, the path will get smoother and easier to traverse over time. Eventually it will become an easy walking trail with clean, wide paths that I can stroll down with little to no effort.
From all of the books I've read on how neuronal connections get made, this metaphor seems apt to me. The first time I read new information, the going is very slow and difficult. I may or may not understand everything, but it will definitely be difficult and I will definitely be exhausted mentally by the end. In contrast, the Nth time I've looked over information, the going is easy. I can skim right through a text and practically guess what is going to be said before I read it. This is analogous to the well beaten path in the metaphor above. The neuronal connections have all been laid down and they fire quickly and effortlessly.
My Process to Learn New Information
To me, learning new information simply requires lots of exposure via multiple voices, styles, and time points. This means that I do read the same thing multiple times, but rarely from the same source. I can count on one hand the number of books I've read more than once. And none of them involved my actual field of TESOL.
For example, the first time I read a second language acquisition textbook, I would estimate that I understood perhaps fifty percent of the information. There were simply too many acronyms and technical words that I could not understand. In fact, the linguistic terminology in many cases is like reading another language. In fact, I consider learning any new body of information to be learning a new language. The first six months to a year in a new field is largely figuring out what the vocabulary means and who the important people are.
However, even reading just one more textbook on the subject of second language acquisition is going to get you eighty percent of the way there. This follows the Pareto principle or 80/20 rule. After two textbooks, I'm mostly learning nuances and filling in the gaps, but the general topography of the field is understood. This seems to be the case with any field I've been interested in, whether it's language, health, psychology, or philosophy. A couple textbooks will get acquainted enough with the field to have a discussion, albeit probably not a debate with any experts in the field.
The interesting thing about reading a handful of textbooks on the same subject is that you will learn there are some things you never did understand, even though you originally thought a topic made sense the first time. When you read a new textbook and the vocabulary is already known from the beginning, it gives you the opportunity to see the forest for the trees as it were and not be so microscopic in your focus when handling the jargon and decoding individual sentences. You can focus on the big picture and not get so caught up in minutia.
When It Comes to Memorizing
So the answer is a resounding, "NO!". I don't read the same thing multiple times to understand it. But, yes, I do read multiple sources on the exact same topic, which obviously have a ton of overlap. This is time consuming, but knowledge is the bottleneck for most expertise. Besides, as you get a better idea of what you're reading, your understanding actually gets better in a non-linear way. You will find yourself understanding more, the more you know.
The major benefit to reading many sources though is that there comes a point where you simply know the material. You haven't spent any time memorizing anything. You don't review certain chapters from certain books, or spend time making notes or flashcards. You simply finish a book and pick up a new one. After a while, you get the feeling that you've read everything in the book before, even though it's a brand new book. At this point, you've probably read enough on that particular topic to move on to something more specific. You don't have to read every textbook ever written on a topic to understand it. You will know when you know!
Taking second language acquisition as an example again, I felt like I was bored due to lack of novelty in the textbooks after about six of them. From there, I moved on to specific areas of second language acquisition or TESOL. I found textbooks on teaching reading and writing specifically. Or teaching conversation. Or pronunciation. Assessment, fluency, linguistics, pedagogy.
You get the idea.
Just move on when you're read to move on. You will know when that time comes. If it hasn't, then keep reading.
There you have it. My method for dealing with, learning, and getting a handle on new information (declarative knowledge). This isn't exactly the same way that I would go about learning a new skill like baseball or painting (procedural knowledge), but there would still be some time dedicated to knowledge acquisition with skills as well.
For information rich fields of knowledge simply:
There is nothing hard about this. It just takes time, discipline, and a genuine interest in the information. If you don't genuinely want to know the information, you won't have the patience to read through five to twenty books on the subject and if that is the case you probably have to wonder what you're doing with your time and why.