A wide variety of sourcebooks have full write-ups. The original House books and the later Handbook series are good sources, but you can find full writeups scattered throughout the FedCom Civil War, Inner Sphere, Shattered Sphere, Warriors of Kerensky, Hot Spots, Touring the Stars, WizKids INN Planetary Profiles, etc.
Plus, there are references to locations, climates, flora, fauna, etc. in the novels, scenarios, short stories, etc. Sarna has been pretty good at catching and compiling those.