The Talmud says that there are 613 commandments in the Torah:
Rabbi Simlai, when preaching, said: 613 precepts were communicated to Moses: 365 negative precepts, corresponding to the number of solar days [in the year], and 248 positive precepts, corresponding to the number of members in a man's body [joints, or bones, covered with flesh and sinews, excluding teeth]. [Makkot 23b]
You would think that the Talmud would tell us next what they are, considering how critically important that is. But it doesn't. Strangely, the Talmud provides no list. Later, commentators have offered many candidate lists, which don't always agree among themselves.
Why did the Talmud not provide a list of the 613? Could they have been in a lost book? Any speculation on that in the Sources?