Why is Tom Bombadil not affected by the Ring in the book The Lord of the Rings?

There is no real answer to this because Tolkien kept Tom Bombadil mostly a riddle to everybody. One explanation Tom Bombadil gives is that 'he is master'. He is in full control within his own borders and nothing can overpower him at least there. But again, most things about Bombadil remain a riddle.
Gandalf said that those with a great enough power of their own can command the ring, Tom Bombadil knows his boundaries and does not try to step over them, so the ring cannot corrupt his mind to evil.