Painstakingly collected over a lifetime by Mayme Agnew Clayton -- a retired university librarian who died in October at 83 and whose interest in African-American history consumed her for most of her adult life -- the massive collection of books , films , documents and other precious pieces of America 's past has remained essentially hidden for decades , most of it piled from floor to ceiling in a ramshackle garage behind Ms. Clayton 's home in the West Adams district of Los Angeles .