The great works tell us how earlier ages imagined the world, but, if they are truly great, speak to us of our lives too.
1. I cannot understand the underlined "but". Is it the same as "not only,,,but also"?
If so, should I say "The great works not only tell us..., but, if they are truly greatly, speak to us of our lives too" instead?
2. Does "of" here mean the same as "about"?