How can a Muslim distinguish what is right amidst the spread of takfiri movements, and how can one ensure their worship is upon the methodology that Allah and His Messenger are pleased with, and what are the simplified books that help one gain understanding in the religion?

Beware of extremism and fanaticism, and of befriending those who excommunicate Muslims. Adhere to thepath of the reputable scholars, such as Ibn Baz and Ibn Uthaymeen, who follow the path of truth and Sunnah, avoiding both extremes. Avoid those who are self-conceited in their opinions and disparage scholars. Benefit from the books and fatwas of the two Sheikhs, Ibn Baz and Ibn Uthaymeen, and from Ibn Uthaymeen's beneficial commentaries on creed, exegesis, hadith, jurisprudence, and Arabic. For creed, read "Majmoo’at al-Aqeedah" by Omar Al-Ashqar. For jurisprudence, "Al-Mulakhkhas al-Fiqhi" by Al-Fawzan. For exegesis, Tafsir by Ibn Sa'di or Al-Jazairi, then the abridged Tafsir of Ibn Kathir. For ethics and conduct, "Maw'izhat al-Mu'mineen" by Al-Qasimi, and "Al-Daa' wa al-Dawaa'" by Ibn Al-Qayyim. Benefit from reliable fatwa websites.