As a young student I did not attend any Islamic school but rather I am reading Islamic books. Why is Islamic book is much harder to grasp compare to Textbooks, how to really grasp what I read in Islamic book?

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There are some good sources on al-islam.org. If you have difficulty following along Islamic books, I recommend you look into lectures and classes. There are several online Hawzas. They would be beneficial for you. For example, hujjahseminary.com offers English classes that you can follow along.