Most of Szass Tam's history hasn't really been that detailed. The best summary of his history is that we know he became a lich about two hundred years ago, and became zulkir of Necromancy. The rest of what we know is mostly in regards to recent events (such as the decision to de-emphasize military expansion and focus on trade expansion).
He was recently featured in the novel The Crimson Gold by Voronica Whitney-Robinson.