Where Alice's Adventures in Wonderland uses a deck of cards, Through the Looking Glass revolves around chess. Alice herself is a pawn who must travel across the entire board to be crowned Queen. Along her travels, she meets both the Red and White Queens, Tweedledee and Tweedledum, a White and Red Knight (who fight over her), and many other characters. While it isn't explicitly stated, she also encounters the March Hare and the Mad Hatter from the first book (but she does not recognize them).
Through the Looking Glass is a crazy adventure. It's worth reading, especially if you are interested in all of the newly envisioned Alice stories (like those I mentioned in my last post), particularly if you are interested in seeing how some of the well known characters like Tweedledee and Tweedledum fit into her story. But overall I enjoyed the first book better.