The best thing I can recommend for learning on Tisha B'Av is the excellent (many people have told me the best they've encountered) class on what Geula and Moshiach mean by my mashpia: "A Vision of the Future" (if you find the quality of the sound too low for understanding, I suggest you use headphones or turn off your air conditioner; or, better yet, download it and listen on your iPod).
Another good class to listen to is one by Rabbi Paltiel: "Diminishing the sadness of Tisha B'Av through simcha".
* * *
My wife and I had the pleasure to spend this Shabbos with my mashpia, Rabbi Yochanan Rivkin and his family, who were visiting New York. During the farbrengen, he spoke about the idea of the number three, which is a bit of a magical number in Judaism: it signifies at the same time permanence (an event repeating three times becomes a chazakah) and a union of the two opposites.
We see this in the idea of the third Beis HaMikdosh. It will be at the same time a permanent Beis HaMikdosh which will never be destroyed, and it will combine the best aspects of the first two Temples.
The first Beis HaMikdosh represented the idea of top–down relationship between Hashem and Jews. The times of the First Temple were the times of prophecy, of miracles (in the Temple), of G-dly revelation (to the point of revelation of Hashem's Essence, which went even beyond the concept of miracles). The problem was, however, that this reward, this relationship were unearned. The Jews would come to the Temple, receive amazing revelation, and still remain unchanged. Therefore, the Jews were free to degrade in their spiritual status, despite the moments of awesome influence.
The second Beis HaMikdosh was reverse: all about the idea of Jews' effort. There was no prophecy, no open miracles, no infinite revelation. It was all about one's work. Which meant that the reward was earned and internalized, but it also meant that a Jew did not receive the infinity of G-d — rather, only a limited degree of revelation that was proportionate to his spiritual level. When the spiritual level of the Jewish people fell low enough, they became automatically unworthy of Beis HaMikdosh.
Because of the two limitations of the two temples — in the first one, that the reward was unearned and not internalized, and in the second, that the reward and relationship were limited and subject to the level of a person and thus also subject to the eroding forces of time and space — both were not permanent.
And this is what the Third Beis HaMikdosh will combine: at the same time, the awesomeness and infinity of top-to-bottom revelation and the internalization of that revelation into the souls and bodies of the Jews themselves due to their bottom-to-top effort.
This is something for us to look forward to and to pray for. But on a certain level, we are also required to bring this about by corresponding to this level ourselves. Now, this is a very difficult thing to do: how do you reconcile the infinity and the finitude?
Nevertheless, this is what is required of us, and this is, as one of the guests at the farbrengen said, to a certain degree the essence of Chassidus Chabad of the seventh generation: to remain someone fully connected to holiness, to spirituality, to Torah and mitzvos, to Chassidus, to uncompromising standards and mesirus nefesh, to ignore the scoffing and the influence of the world outside — and, at the same time, to remain connected with the world, to the point of not only interacting with it, but also influencing it.
One could say: this is a very difficult thing to do. Rabbi Rivkin told a story of Rabbi Segal who complained that the assignment given to him by the Rebbe was very difficult. The Rebbe responded: "Since when did you make a contract with the Almighty for an easy life?"
Point being: we are here for a purpose. If you want to just live your life away, not worried about anything except yourself, fine. You can do that. And I don't mean eating cheeseburgers. An intelligent person recognizes the truth of Torah and the value of mitzvos for himself, for his neshama. So, one can learn Torah, do mitzvos, live in a wonderful community, enjoy his life, both physically and spiritually — and what's wrong with that?
What is wrong is that we are still in golus; Beis HaMikdosh is not here; Moshiach is not here, and the purpose for which Hashem created this universe has not been fulfilled.
So, the Previous Rebbe said: In these times, called the heels of Moshiach, everyone must ask himself: "How am I contributing to the coming of Moshiach and geulah?"
And his follower, our Rebbe, answered that question for himself and gave each of us an opportunity to answer it as well.
P.S. See also this article about the number three in Judaism.