According to a culling of Earth's old information network, Petrenka's secretary, Jean Dufresne, had been given an "executive order" to transfer to a similar podiatry office on Mars for "clerical duties." He worked at a military campus where he had sent the package from when he arrived. Petrenka sent him a package in return, but it looks like no more mail was exchanged between them. Perhaps her package remains at the Mars campus?

Maybe the darkness arrived? The war began

Yes, all non-essential personnel on Mars, Jean included, were sent to the Ishtar Academy on Venus. Transit between planets was almost entirely shut down as the Darkness overtook the system, and the few transmissions sent after are mostly Warmind status reports and warnings.

I hope jean or her omnidirectional family are still around.. Maybe your guardian wants to return the book maybe find out if the original owner is still around or maybe the owners children?