I believe that for the average person and reader, they would have to read a book several times before they are able to fully and completely understand its contents for themselves.
Reading a book just once is most often very insufficient and very inadequate for any person of average intelligence to be able to gain a total comprehension of its contents thoroughly.
Obviously, the more times that any person or reader were to read and study a book, the greater and deeper the level of understanding and comprehension will be.
You just have to give your mind the necessary time to fully and completely absorb all the contents of any book that you read.
It does take a lot of patience, a lot of focus, a lot of determination, a lot of time, a lot of effort, a lot of money, for any average person to be able to effectively learn any highly advanced and highly complex subject within a reasonable period.
This is because all the different highly advanced and highly complex subjects require a lot more time, a lot more effort, higher levels of intelligence and higher levels of logical/ rational/ critical thinking, to be able to let any average person really have a very good grasp of its many different highly abstract and highly complicated concepts and theories.