This is a preprint of the book with the same title which has not yet taken its final form. Chapter 7 and the part of Chapter 6 dealing with the continuum hypothesis are not included because I am still working on them. I am also planning to include much more exercises than can be found in this edition.
I appreciate the discussions of the early version of parts of this book which I had with professors Karel Hrbacek and Michael Anshel of the City College of New York. I am much obliged to Susan Goeckel, Robert Nirenberg, and James Piccarello for their help in editing.
The work on this book was partly supported by the National Science Foundation grant # MCS-8007565.