How to effectively study Islam, is there anyway to increase memory and ability to understand, because when i read something I usually forgot it

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There are many books where you can find the real message of Islam or you can have true knowledge of Islam.
Please read this book

https://www.al-islam.org/shiite-anthology-sayyid-muhammad-husayn-tabatabai

There are many books you can find regarding the life history of Aimmah (a.s) or biography of 12 Imams (a.s).
One of them which covers both biographies and their Socio- political life is “ The Lives of the Twelve: A Look at the Social Political Lives of the Twelve Infallible Imams”
Written by: Shaykh Mahdi Pishvai
Translated by: Sayyid Ali Musawi.