Shaarei TeShuva For Lashon HaRa
Rabbeinu Yonah in Shaarei TeShuva Gate 3 (Gates of Repentance) goes over the vastly complex laws of Lashon Hara. Not everything that sounds bad is forbidden, and not all nice speech is permitted. One of the worst forms of speech is flattery and praise of the wicked. It's important a person learns the laws so they don't commit sins and so they don't judge righteous people as wicked, which is also a HUGE SIN.
