Cathechism
Cathechisms are instructions basic to the catholic faith. If you are a catholic, but has not received proper instruction about the faith, than this is for you.
You will get a better sense of how to be a good and devout catholic, and what does it mean to be one. Do you know that you are God’s Chosen People? Yes you are, and you are called to become great saints as well, no matter how many or how big your sins are, the mercy of God is infinite, and he will make you a saint, like St. Dismas (the thief next to Jesus), St. Mary Magdalene, St. Paul, St. Margaret Cortona, St. Pelagia, St. Mary of Egypt (the last two are ex-prostitutes), if you go to confession frequently, He will make you a saint. And you will have deep loving relationship with Him, as if no one existed. Because he loves you. Personally, and he knows you.
Cathechism of St. Pius X book
A must have for every catholic. Very simple and understandable. Not lengthy but short Q and A. Unlimited Stars, haha
Cathechism on the Catholic Dhuay Rheims Bible
The only one and true bible, with simple words, and profound in meaning. Translated with approval of the Holy See, not on person to person basis.
Cathechism of the Catholic Church vatican.va
This one is quite lengthly, but if you are determined to know what it means to be a catholic, human sexuality according to the will of God, and so on, go for it.
Enchiridion of Indulgences book
After you make a good hearty confession, if you do not want to go to purgatory to suffer without merit, maybe for 5000 years because of mortal sins… (I’ve heard that, even more, I think… ) This is the book. The plenary indulgence gives you a release from all temporal punishment, if you fulfill all the three conditions, sacrament of confession, eucharist, and prayer for the intentions of the Pope (1 Hail Mary, 1 Our Father), and no attachment to any venial sins whatsoever, this can be done by asking God the Grace (God, disattach me from any sin mortal or venial.), and the work required, such as recitation of the rosary in a church, adoration of the blessed sacrament for half an hour, reading of the Scriptures for half an hour.
And you will also get a plenary indulgence on Divine Mercy Sunday (that is, first week after easter), if you go to confession and receives eucharist, with no other requirements other than a hearty contrition and sorrow for your sins. Isn’t God’s Mercy unfathomable?
For your information, 5000 years in purgatory is better than be in hell, for eternity.(that means 5000 years* 1 billion years * infinity) It’s not to late, do not postpone God’s Call. You are created only for Him, to love Him now and for eternity. God doesn’t see studies, riches, honors, money, house, cars, popularity, name, beauty, he only sees your heart. Your suffering heart, and he has compassion for that. The mercy and compassion of God for you no one can imagine. He misses you. You are precious in His heart. As a father misses his son who is missing. He doesn’t judge you. He knows your pain, that you are hurt, by other people, by family, dissapointed, but he is the best of fathers, and mothers. He is unlike earthly father or mother. No, He is Love and Mercy itself. And he wants to heal your heart with His holy love.
You have to let yourself be loved by God, and ask his mercy, for you and the whole world.
Catechisms of St. Thomas Aquinas
You got the basics.
Now go on, be a pro

(a pro in catholicism means the most humble, the lowest of all, but exalted high in heaven, and gain many merits for the Glory of God and Salvation of souls)
Saints Books
Autobiography of St. Gemma Galgani (webpage)
St. Gemma Galgani is an italian saint, lived in around 1880, died and attain sanctity at a young age, in her twenties. She went to confession three times a week and went to mass and received the eucharist three times a week
Diary of St. Gemma Galgani (webpage)
Very interesting account of St. Gemma Galgani’s life, in her own words.