Assalam o Alaikum wa rehmatullah wa barakutuhu, Muqaddimah or Introduction of Sahih Muslim. It explains some of the most important aspects and principles of hadith science, importance of sahih hadith and fitna of zaeef, mozzu hadith. Imam Muslim Neshapuri wrote this entire book for the benefit of common muslims, as he himself said in the introduction. Every muslim must know atleast these rules and principles, so that he may remain safe from the fitna of zaeef, false and fabricated ahadith, which is now a days very common issue, among both sunnies and shias. It's a must read document. It will surely remove many of your doubts.
Read online: http://sunnah.com/muslim/introduction
Download (English): http://www.kalamullah.com/muqaddimah-sahih-muslim.html
Download (Urdu): http://ahlesunnatpak.com/research-papers/ (Paper no. 15)
جزاك اللهُ خيرًا