The book that changed my life was the "Bible". If you are going to read it by yourself it maybe hard for some text to understand. Another book that made me understand the Bible clearly is with the help of another book "What Does The Bible Really Teach?" It was published by Jehovah's Witnesses. For the passed century they're not stopping studying the Bible and what does it really teach. For the second book I'd mentioned, it could answer major questions that relates to all people. It could answer questions like "What is the truth abou God?", "What is God's purpose for the earth?", "Who is Jesus Christ?", "Where are the dead?", "Are we living in the last days?", "Why does God allow suffering?", and a lot more. It's easier to understand reading the book "What Does The Bible Really Teach" if you're going to read it together with the Bible. It has some explanations in which it will tell you where in the Bible you can find it. The best way to learn about the Bible is if someone who is more knowledgable will teach you. On the website https://www.jw.org you can read the Bible online, you can also download and install the app so that you can also download the Bible as well as the other book I've mentioned and a lot more books and magazines. You can download any of those for free, there's also some videos which is very useful in our everyday lives. The best of all, you can also request on the website for a free Bible study on the comfort of your own home. Everything is for free. It changed my life and for sure it could also change yours. It's available worldwide on different languages. Again here's the link, https://www.jw.org