You should get around to book 1, its not particularly long, and its a classic

The Second Foundation is basically a SPOIL ALERT (i guess) a hidden, secret society that is is many ways the opposite of the First Foundation, though with the same goal, which happens to be saving the universe.
Its kind of similar to the yin/yang of Dunyain/Cishaurim. One physical and emotionless, the other more nebulous/metaphysical and emotional. Both required to save the universe, but not to meet until the time is right. Second Foundation knows of the First, but the universe (and the other Foundation for that matter), either doesn't know or doesn't believe in the existence of the Second.