Theory of Computation is a text for a first undergraduate course. It is Free. Use it as a main text, as a supplement, or for independent study.
Get a printed book
You can get a paper copy. Click on the Amazon link below.
Are you teaching with this?
You may find useful a number of ancillary materials, including classroom projection slides and a machine simulation programs.
Note that there are differences between the printed version and the version available online. Some are minor, such as typos corrected in the online version that have not yet made it to the printed version. And the online version has some animations, which of course are not in the printed version. In addition, for license issues, a small amount of material in the online version does not appear in print (and consequently the page numbers differ a bit).