These books explained Judaism to me, helped me with my conversion decision, and continue my development as a Jewish adult.
Choosing a Jewish Life: A Handbook for People Converting to Judaism and their Family and Friends by Anita Diamant - An excellent resource. Talking to a rabbi about conversion isn't actually that scary, but if you want more information before that step, consider this book.
The Everything Judaism Book by Richard Bank - Easy to read, with lots of sidebars for additional information.
To Life! by Harold Kushner - I hated this book when I first read it, because I felt Kushner was being pushy about taking on more mitzvot. As I gradually started to add more Jewish traditions to my life, I found that Kushner was right; they did make me feel more comfortable with Judaism, even if the mitzvot themselves felt strange at first.
The Sabbath by Abraham Joshua Heschel - I highly recommend all of Heschel's books, but The Sabbath is by far the best place to start. It will make you think about time and place in a completely different way.
As A Driven Leaf by Milton Steinberg - An excellent piece of historical fiction set in the Hellenistic Period (1st-2nd centuries CE) that deals with the conflicts between logic and religion, doubt and faith.
Doing Jewish Theology by Rabbi Neil Gillman - Rabbi Gillman covers some extremely interesting topics, including the Torah, halakhah, and chaos. Read my full review.
To Be a Jew by Rabbi Hayim Halevy Donin - You probably don't need to know how to kasher your own meat, but the chapter is a fascinating read anyway. Besides that, Donin covers every other topic you could possibly want to know from a traditional Orthodox perspective, in a way that makes it accessible to people not coming from that world. It offers great explanations of Jewish tradition.