The Bible mentions a few things about the sadducees' beliefs, such as in Acts 23:8, but doesn't reveal much detail. Are there any documents aside from the Bible from the first century AD (or first few centuries) which document what the sadducees believed?
Compared with the Pharisees, very little is known about the Sadducees in the Herodian period. The cause of this marked contrast is not hard to find, as Helen Bond points out in her book Caiaphas. In the upheavals that followed the two revolts against Roman rule (66-70 and 132-135), the Pharisees emerged as the winners and the Sadducees as the losers. There have been (in Bond’s words) “generations of Jewish scholars who understood Pharisees as the correct and trustworthy side of Judaism. … Unlike the Pharisees, however, the Sadducees have no modern successors with an interest in rehabilitating them. ... The Sadducees left no written records, and we are forced to rely instead on the negative parodies penned by their opponents.”