Theory of Computation, is a text for a first undergraduate Computer Science theory course. It is Free. Use it as a main text, as a supplement, or for independent study.
Extra materials
As noted on the description page, the fully-worked answers to all exercises is available.
If you are teaching from the text, you may find useful the slides for classroom presentations. These cover the same material as the text, in the same order. They are made using the book's LaTeX source so they are sure to have the same wording, notation, etc.
There are also videos based on those slides.
Suggested additional readings
These books are available at most college or university libraries.
The Universal Computer: The Road from Leibniz to Turing, by Martin Davis (ISBN-13: 978-0393047851) is a wonderful historical overview of the development of the subject, written for a wide audience by one of the central people in that development.
Gödel, Escher, Bach: An Eternal Golden Braid, by Douglas Hofstadter (ISBN-13: 978-0465026562) is a charmingly discursive exploration of many of the ideas of the course (it is also a Pulitzer Prize winner).