I just finished studying elementary first order logic through school course, and I am looking for some good books for studying more of mathematical logic. I especially pursue to learn about formal system - (which was used for incompleteness theorem, and noted in a book by Douglas Richard Hofstadter.)
I am thinking of having book "A Profile of Mathematical Logic(written by Howard Delong)", but I'm not sure if this book is suitable for me.
Thanks for any advice for this.