Bet is the second letter and has the value two. Its name is that of the house, bayit, and Sefer HaBahir says so plainly: "do not read Bet, but bayit." Its shape is closed on every side except the front. It is the first letter of the Torah, that of bereshit, "in the beginning." Sefer Yetzirah places it among the letters of the lips, with Vav, Mem and Peh.
In Sefer Yetzirah, Bet is the first of the seven doubles, Begad Kaporet. Like them, it is pronounced in two ways, soft or hard, and the book sees in this "the pattern of soft and hard, the pattern of strong and weak." G-d "made the letter Bet reign over life and bound a crown to it," and through it formed Shabbetai, the planet we call Saturn, in the world, the first day of the week in time and the right eye in the body. Each double carries its opposite: that of life is death.
In tractate Shabbat, the children of the house of study read the first two letters in a single breath: "Alef Bet, alef binah," learn understanding. If Alef becomes the command to learn, Bet gives binah, understanding. The two letters do not separate: study and understanding go together.
The Midrash Bereshit Rabbah asks why the world was created through a Bet. Rabbi Yonah, in the name of Rabbi Levi, answers: just as Bet is closed on all sides and open in front, you have no permission to ask what is above, below, before and behind, but only from the day the world was created. Other answers: to teach you that there are two worlds, this one and the one to come; and because Bet is the letter of blessing, berakhah. The Bahir adds that Bet is a house for the world, for the Holy One, blessed be He, is the place of the world and the world is not His place. The Alphabet of Rabbi Akiva also says "do not read Bet, but bayit," and has G-d speak: "I built two palaces, one above and one below."
Two, the house, the blessing. Bet opens the Torah facing forward, and the Sages draw from it both a limit and a direction: not to claim to probe what precedes creation, and to dwell in the world that has been given. In Sefer Yetzirah, it carries life; for the children of Shabbat 104a, understanding. The house begins with what one learns in it.