Question: If we have the Jewish law all laid out for us in contemporary books, then why do we find it necessary to study talmud? Isn’t it an early form of Jewish law ?
Answer: That is a very reasonable question, but – we certainly do not have the law all laid out for us! Halacha (Jewish law) is a complex continuing process. There are many different opinions by many different experts, and they are in turn based on weighing various different opinions from the past. When a new question arises, or a situation that has a variation, it has to be done again to bring things up to date.
And all of this is entirely based on the Talmud. The basic rules and understanding are always derived from there, and the later discussions are always an attempt to apply those principles to new situations.
Why the Talmud? Till a couple of thousand years ago, we had a Sanhedrin, the Jewish Supreme Court. When a question would arise, it would eventually get referred to them, and they would either have a tradition on the answer, or would work it out from the Torah. What they said was final (Deuteronomy 17(8-13)).
Since then, we had no Sanhedrin, but we did have the Babylonian Torah academies. They were accepted as the main authority by all of Israel. When their deliberations were compiled into the Talmud, that too was accepted; it is our last contact with the authority of a Sanhedrin.
Anyone who wants to be a posek, an expert on deciding questions of Jewish law, must first be an expert in the details of the entire Talmud.